I wasn’t in a fraternity personally, but:

Fraternities are basically social clubs (some all-male, some all-female - called ‘sororities’- some co-ed) that host a range of social activities - parties, events, etc. etc. They can be quite a central feature of social life at a lot of American schools and are pretty iconic of the “American college experience”. Some colleges don’t have any fraternities, while others (especially Big 10 Schools or large State schools) have a very active “Greek Life”. I went to Harvard which has a mixture of fraternities and “final clubs”, though a lot of the social activities take place outside of these groups as well.