Definition of overbet
An overbet is a bet that is larger than the size of the pot. If you bet 15??? on the flop in a pot of 10???, you have made an overbet. An overbet is only called after the flop and almost never preflop. Even if a player open-raises to 48 big blinds preflop, it is not really called an overbet. The overbet is identified with the size of a pot. It is mostly used by good players for a variety of reasons.
Reasons to make an overbet
You can overbet for any number of reasons. You can overbet with a very good hand, you can overbet in a bluff, you can overbet to merge your range, etc. Let's say you are on the river and the river card completes the flush. Here, you suspect your opponent has a pair at most. You have nothing. If you bet normally, you're afraid he'll call the bet without much thought. By overbeting, you give him something to think about and perhaps discourage him from calling the bet. Another good reason to overbet is if you have the nuts on the river and you suspect your opponent has a very strong hand. Here, even with an overbet, you know he will call, which will maximize your profits.