Definition of overcall
An overcall is a "call over". An overcall is a call that is usually made preflop after another player has already called. If the UTG player raises preflop, he opens raise. If the player on the hijack calls, he cold calls. If the player on the button also calls, he overcalls (a call over a call). Generally, the overcall is not as strong as the cold call, but still shows some strength. A player may want to overcall because there are already several calls and the pot size is getting interesting. Also, if he is in position, he will have even more reason to overcall.
It is better to overcall in position
Overcall is a risky game. If you overcall, there are at least two players already involved in the hand. A skilled player in position or in the blinds might see this as a good squeeze opportunity. If you overcall out of position, your overcall will probably lose in the long run since you won't have the initiative, you'll be out of position, you'll be multiway and you probably won't have a very strong hand preflop. ?? do it sparingly and especially in position.