Re: What is metagaming
I was involved in improvisational theater for a while. It's theater, so you're playing a character, acting like that character, and trying to be convincing at whatever it is. In that sense, you have to be authentic, and you have to separate your own knowledge and preferences from those of the character.
But it's theater so you're also playing a character in a scene that hews to certain dramatic conventions. Some of these are technical, like projecting to the audience and meeting the room's general standard of taste, but some of these are purely narrative and focused on not just reacting realistically, but reacting realistically in a way that lets the scene go somewhere, to have a beginning, middle and end. I'm sure there are avante garde troupes who are happy to get on stage and act out a boring party, or sitting and watching TV, or sleeping, or having a pointless argument, but mostly good scenes go somewhere.
One of my improv teachers called this "puppet and puppet master." One has to be immersed, but one also has to know what's going on at a "meta" level. Most acting is like this: actors want to be believable, but they've also probably read the whole script. They know what's going to come next and not only do they have to react realistically when it happens, they have to work it make it happen in a realistic way. They can work out kinks along the way, such as pointing out that the audience would think that the character would know something that they appear not to, or vice versa, but the goal isn't to "win," it's to make a good story.
I know that a lot of people who roleplay do so to become completely immersed, to not have to think in terms of being a "puppet master." They don't want to have to consider the rules, or the existence of the other players, or real-life interpersonal relationships or social norms, or anything. For them, the most important thing is that a choice be not just plausibly what the character would do, but undeniably what that character would do. Anything out of character feels jarring to them, because it forces them to see the unreality of the situation.
Some GMs rely on this, which they can do because they know the players want it. The GM can then set up a situation that anyone might see right through, and the players walk into it (or don't) not because it's dramatically appropriate, but because it's in-character.
So, given the aforementioned, "meta" thinking of any kind becomes almost a breach of contract, and a hardship on those involved, especially if the meta-thinking stems from out-of-game desires like benefitting one's own character, getting back at someone for a slight, or trying to avoid looking dumb. It forces everyone to be aware of the meta level, and have to counter such tactics. Metagaming is, in this setting, bad.
At least, that's my understanding. I don't play that way myself, but I've met some people who have. I was once almost reported to the moderators on a forum because I pointed out to someone that the game wasn't real. Having that thrown in their face was, to this person, harassment, in that it ruined their ability to enjoy the game. I'm not even saying that they're wrong for thinking that way. They have what I've heard called "the right to dream," and if I insist that they not "dream," or do things to hinder it, then I'm the jerk.
That's a somewhat extreme perspective on the game, though. Maybe it's the ideal for many people, but I know that a lot of people play it on a different level. Lots of people don't play it to emulate reality, or react realistically, but to emulate movies or stories. This approach almost requires metagaming. If one is playing a silver-age-of-comics-type game, then the when the good guys win, they hand the bad guys over to the cops, even though the last five times they did that, the bad guys later got away and terrorized the city again. An immersed player would act as a rational person would, and do something different this time or, heck, probably the second time.
A player in more of a "puppet puppetmaster" mode would let their character do the irrational thing, because they know it's part of the convention of the genre they're in. And maybe that's a big part of it: immersed characters, whose players aren't metagaming, are "genre blind." They don't know they're in a hard-boiled detective story, because they're in whatever story they make it to be. Maybe they ignore the central crime entirely; maybe there is no "central crime," just one of a large number of events going on, and maybe the character has a clear imperative to do something else.
(Another analogy is with model train makers. The trains don't necessarily "do" anything, the point is that they are accurate to minute levels of detail.)
Players who are more interested in the meta-story, and of emulating a certain genre, might have to metagame, to bring that about. If a game is "about" going into dungeons and fighting dragons, then that's what the players have to arrange for their characters to do, even if, based on everything one might learn from the NPCs and history books of the setting, that's a really, really bad idea. Players who are keen on getting as much plausibility as possible into their situations might have to come up with reasons why doing this really foolhardy thing is at least plausible, if not truly justifiable.
Oddly, metagaming can amount to its own form of immersion. I find this is the case whenever I decide not to look too closely at a character's motivations. Informing me that my character wouldn't do the thing I want him to be doing becomes the jerk move, because I have a different motivation that just faithful emulation of my character as a real person. I know and embrace the fact that my character isn't real. I can gleefully send them into danger or let them be killed - or kill them myself. The character is there for my amusement. I'm still "roleplaying" because I'm having the character do things they'd do, but at another level I'm working with the others at the table to ensure that my character wants to do things that I'm interested in.
(The "model train"-type analogy there might be people who, for instance, make drones that look like Star Wars vehicles and have dogfights in the style of the movies.)
On the far end, are people who are purely in the mindset of the game as a game. Playing it like a modern cooperative boardgame, which can be a lot of fun. The jerk move at a table like that is to tell someone that they can't make a great move, even though it's entirely legal, because it's against the theme of the game, or out of character.
("Model train" analogy: people who make RC vehicles that are souped up to win something or do something well, not designed around real vehicle specifications.)
Long story short, sit at a table that's right for you. People who want deep immersion have to know that that's what everyone else wants - or at least that everyone there is dedicated to enabling that immersion (people get immersed in movies, even though no one actually involved in making the movie was immersed). People who want cool stories need to know that everyone wants cool stories, of approximately the same kind. People who want to make ideal logical choices based on a high-level view of the situation, need to sit with like-minded people. All of those are reasonable and fun ways to play, but mixing them can result in one set of people thinking the other sets are jerks and vice versa. True jerks are actually pretty rare, I find.