A very weak notrump opening bid to show a balanced hand of 10-12 HCP. 2C response is Stayman; Jacoby and other transfers are used where partner has weak hand. Some players use 3 level bids as natural and game-forcing.
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.