On the auction to the right, North's jump to 2NT shows a hand too strong to open 1NT and too weak to open 2NT. North's jump to 2NT typically shows a balanced hand with 18-19 HCP. In this example, South responded 1♥. Since North would have raised hearts with 4-card heart support, North's jump to 2NT denies 4-card heart support. North may have 4 spades on this auction.
Responder, South, may want to stop in 2NT or 3 of a suit. Responder may want to play 3NT or to look for a major suit fit. Responder may want to offer a choice of games. Responder may want to look for a slam. Tools are needed to accomplish responder's goals. Many partnerships play New Minor Forcing over opener's 2NT jump rebid. The problem with New Minor Forcing is that it doesn't allow you to stop in a suit at the 3-level, and doesn't offer many choice of game auctions. The Wolff Signoff solves these problems, but is complex. Playing transfers is easier and better.
It is easy to stop in 3♦, 3♥, or 3♠ playing transfers. Responder bids the suit below the desired strain. Opener is forced to bid the next suit. Now responder can pass if they wish to play there. To show extra length in responder's original suit, responder bids the suit below. Responder can then offer a choice of games or place the contract. To show slam interest, responder uses a transfer at the 4-level. Clubs are an exception. An artificial 3♠ rebid by responder shows clubs and slam interest.
For example, in the auction to the right, responder's 3♦ rebid is artificial, showing 5+ hearts. Opener's 3♥ bid is forced. Now responder's 3NT bid offers a choice of games. Opener will pass 3NT with a doubleton heart and bid 4♥ with 3-card heart support.