In the fields of sociology and social psychology, the terms ingroup and outgroup refer to the groups that individuals use to build their personal identity. The ingroup is a group that you belong to, and it's important; your race, politics, sex, gender, religion, and nationality may all be relevant groups... and not surprisingly, these generally appear in oppositional groups, with each ingroup having at least one outgroup. You are christian and definitely not hindu or muslim; democrat, and certainly not republican; male, and not at all a sissy; American, and not a damn communist.
Humans are really good at creating ingroups and outgroups, from Chelsea vs Arsenal to the Communist Party USA vs the Freedom Socialist Party to the Independent Fundamental Baptists vs the Southern Baptist Convention to Team Mystic vs Team Valor. While these divisions seem trivial -- or imaginary -- from the outside, the suggestion that you switch from Your Thing to The Other Thing is likely to strike a member of either group as disingenuous... or even insulting.
In recent years the terms ingroup and outgroup have been co-opted by the rationalism community, where they mean approximately the same thing but with a somewhat more judgmental edge to them. The term entered rationality circles by way of the in-group bias, a cognitive bias in which one automatically tends to give preferential treatment to ideas that come from people within your own group. As a significant part of rationalism consists of identifying and eliminating cognitive biases, rationalists are less understanding and tolerant of in- and outgroup creation than is your average sociologist.
While it is recognized that everyone has ingroups and outgroups, rationalists are encouraged to recognize the groups as arbitrary when they are, unimportant when they are, and flexible when possible. If a group is a stable ingroup, it will usually start to shut out the outgroup, but it should be recognized that the existence of an outgroup generally indicates that there are a group of people for whom the ingroup's values and actions are insufficient.
As you might expect, the out-group bias is a useful bias to refer to frequently... and during election season, nearly constantly. It's easy to come up with political examples; just look at any political meme that comes your way. It is impossible to keep up a steady stream of short-form but meaningful insights in a highly repostable format; but that's okay, your ingroup won't mind if you drop the meaning and just focus on being witty and/or angsty. You will probably not have to look hard to find a serious post comparing any given candidate to Hitler. It is surprisingly easy to disdain this when your opponents do it, and startlingly apropos when your friends do it.