An AGM is the once-a-year meeting of a club, company, or organisation. Members hear what happened last year, look at the money, ask questions, and often vote for the people who will run things next year. The letters stand for Annual General Meeting. You will see AGM on invitations, company notices, and club emails; it is office-and-committee English, not street English. British and international groups use the abbreviation a lot; American speakers may just say "annual meeting." Though actually, those same three letters can mean other things in other worlds, so if you are not looking at a club or a company, check the rest of the sentence before you decide.