An opening of two spades or two notrump shows a weak hand with a long suit (minimum of 6, but usually 7+ cards).
Response:
(a) if 2S was opening, bid 2nt to learn long suit
(b)i if 2nt was opening, bid 3C to learn long suit
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.