Bids of suits that show the suit ranking immediately above the one bid, and ask partner to bid that suit. They are most common after partner opens 1NT or 2NT and allow greater flexibility in the bidding.
The basic principle is that with, for example, a five-card or longer heart suit, one responds in diamonds. Partner will convert to hearts after which responder may pass with a weak hand, or make some further descriptive bid.
An elaborate system of ‘transfer bidding’ sequences can describe a range of hand-types. For example, to make a 2♦ transfer in reply to 1NT you might hold:
With the first hand, you intend to stop in 2♥, playing in a safer partscore than 1NT while keeping the stronger hand as declarer. With the second, you intend to rebid 3♣.