Ham is meat from the top of the back leg of a pig, specially treated so that it can be kept for a long period of time.

A ham is a person whose hobby consists of using special radio equipment to talk to other people with the same hobby, often people who are in other countries.

A ham actor is someone who acts badly, exaggerating every emotion and gesture.

If actors or actresses ham it up, they exaggerate every emotion and gesture when they are acting, often deliberately because they think that the audience will be more amused.