What is this thing called "Meetup" that everyone keeps talking about? Does it have some meaning beyond the obvious that I'm unaware of?

It's mostly what it sounds like, once you take into account that it's short for "LessWrong meetup" ('meetup for LessWrong users'). The possibly non-obvious bit is that meetups are often recurring things with people who consistently come to most instances of them in a particular area, so they're more about ongoing socialization/skillbuilding/etc than literally meeting people.