Gate keeping is a policy of excluding others from membership or participation in something that one is a part of. While the reason for doing so can vary greatly the one most likely to be cited is that the those gate kept don't belong in the scene because they are posers or newbs who don't understand or would dilute it. While group self selection is a basic feature of association it's often the case that certain people aren't satisfied with organic development and feel the need to try and exclude others. Other gate keepers simply want to aggrandize themselves at the expense of others by discouraging newcomers and keeping their group exclusive. More motivations may exist but the effects remain mostly the same. The gate keepers constantly test, tease, mock, and dismiss those who don't meet their standards or conform to their vision. This can be a rabid sports fan who attends a ton of games and dismisses the homebody TV watching fans, the trekkie who mercilessly quizzes others on every piece of Star Trek media ever released, and the goth who inspects the outfits of every person who comes into the club and rates them out loud.

Gate Keeping is usually considered a bad thing. It discourages new fans from coming to the hobby or fandom. It makes the fan base look like toxic assholes. It just wastes time on true fan arguments which are literally undecidable. In this sense gate keeping is antithetical to a healthy environment. You know what else makes for an unhealthy environment? Having your fandom flooded with misogynist incels, alt-rightcryptofascists, and worst of all milquetoast normies. Having standards and moderation is a near universal feature of online and real world spaces and this is good and proper because we know what the alternative is. The problem is that the difference between censorship and having standards is a thing that each person knows deep in there heart and different people have different hearts. For that reason most everyone is likely to agree that gate keeping is bad and draw the line on what it is slightly differently. Make sweeping claims about the fandom without pedantic qualifiers: gate keeping. Correct people about factually incorrect statements: gate keeping. Having an opinion: gate keeping. Telling off fools: not gate keeping. Speaking out against bad ideas: not gate keeping. Telling people when they've completely misunderstood the themes of a work based entirely on your own interpretation of one line from the author taken out of context: not gate keeping. Humanity's capacity for myopic hypocrisy is unlimited and terrifying.

Possibly the best example to draw from is this very site since E2 is unfriendly to new noders. Gate keeping is a fairly core human behavior and failure mode which is likely to only get more persistent, detectable, and contentious in the future as the ubiquity of anonymous online interaction diminishes civility while leaving permanent records of its occurrence in cyber space. On the other hand the whole people being jerks on the internet problem may just be one of those issues that's best solved by ignoring it. Don't feed the trolls has been a standard refrain for at least two decades now. The problem of elitism in subcultures probably dates back as far as the invention of writing and it's up to each of us to decide how to deal with it when we encounter it and not to perpetuate it.