An opening bid of 1nt shows either 10-12 or 16-18 HCP balanced. Partner bids 2C to learn more about the holding and strength. Opener bids at the 2-level to show the weak hand, and at the 3 level to show stronger opening.
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.