It's a social gathering. Of a specific group of people. The difference between a party, and a group lunch, is basically the time frames, and a more curated group of people tend to go to a party. Whereas a lunch tends to be who's available.

Parties tend to be some celebration, which is a human social norm, but really the reason for the party is less important than the social interaction. Humans need periodic social contact to maintain relationships, and parties are a good avenue for that.

There's a whole spectrum of parties, there's dance parties, there's drug parties, there's alcohol parties, there's board game parties, there's beer parties, there's dinner parties, there's anything. Any form of human social engagement could be a party.

Just think of parties as a way to maintain your social standing, and refresh relationship status, with a group of related people at once. So it's very efficient in that regard.