Eternal September refers to a period beginning in September 1993 when access to the Internet greatly expanded, changing the culture of many internet forums.
More generally, Eternal September sometimes refers to a phenomenon where an Internet community can become worse, different, more watered-down, or less dedicated to its original purpose as it gains more users.
This phenomenon has been observed with respect many communities, for example, to small subreddits that grow dramatically in a short period of time due to some set of circumstances. A community that was initially formed by hardcore enthusiasts for deep discussion can become overwhelmed with lazy and shallow discussion when flooded with people who are not passionate about the subject.
In addition to the above listed examples, an influx of new users can change the culture of a community from jovial and unserious to vindictive, or from ironic and self-aware to non-ironic. A community started for merely discussing the absence of something ("I don't drink coffee") can turn into a community dedicated to hating that thing ("I hate coffee!").
Gatekeeping is the practice of limiting who has access to certain resources, discussions, or platforms.