College General Meetings, are, as the name suggests, meetings that take place a couple of times per term for all members of the Junior Common Room (JCR) to attend, with a fairly general agenda, which members of the JCR - i.e., all the undergrads (who don't opt out of membership) - can add to by proposing motions. At the GM, the Vice President of the JCR oversees all of the motions, and various members of the JCR can pitch in with questions or amendments throughout the course of the meeting.
At my college, the GMs usually take place on a Sunday evening in the college JCR. The Vice Pres also usually gets some free pizza for the start of the meeting. The agenda is sent to the JCR a few days before, and if there are motions on it that people are particularly interested in, more people tend to turn up!
It can be nice to go to GMs as you get to see another side of college, and quite often you get to see people who you don't usually see, for example, if they don't actually live in college or don't go to Hall. Watching the debates and listening to the different ideas people have can also be quite interesting!