After opener's weak two bid, responder bids 2nt to learn about the quality of the hand. Opener bids as follows
3C = 9+ losers
3D = 8 losers, bad hand
3H = 8 losers, normal or good hand
3S = 7 or fewer losers.
A bidding convention is an agreement between two players on a different meaning of a bid in the card game bridge. These agreements are made so that partners can more quickly determine which contract, with both their cards, they can win together. Often, conventions are forged together into one, which is then called a bidding system.