1. The first bid by a member of the side that did not open the bidding.
2. To make a bid after an opponent has opened the bidding.
North’s 2♣ is an overcall and, in bidding 2♣, North is overcalling.
An overcall can serve many purposes:
1. To disrupt the opposing bidding.
2. To suggest a lead.
3. To find a good contract in which to play.
4. To pave the way for a sacrifice, which may be cheaper than letting the opponents play in their contract.
Shape and suit quality are more important for an overcall than they are for an opening bid. Here, North could hold a 9-count with ♣K-Q-10-x- x-x and an ace on the side, but should not hold a 2-3-3-5 13-count with ♣A-J-x-x-x.