A cue-bid in the enemy suit, after partner has opened one notrump and opponents have overcalled. The cue bid asks partner to bid a 4 card major.
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.