When someone is said to 'ham up', it means they're being overly theatrical for attention. Some other synonymous phrases for this behavior include 'playing to the peanut gallery', 'showboating', 'milking it', 'overacting', or 'chewing the scenery'. Sometimes people will 'ham it up' to get more laughs or create a memorable performance, but it can also come across as fake or insincere. Regardless, it's a term that's often used in the entertainment industry and can apply to anyone who's trying a little too hard to grab the spotlight or get noticed.