The Jacoby Transfer is used when a play opens with 1NT and his partner holds at least one five-card major suit (Hearts or Spades). The responder naturally wants the contract to be in his long major suit. With a weak hand he prefers to have his partner play the hand so he indicates which major suit he holds the five card major. He wants to make sure that the weak hand is the dummy, so as not to expose the high cards in the opener’s hand.
The Jacoby Transfer works like this (in response to an opener 1NT bid)
2 Diamonds Shows at least five Hearts; asking partner to bid 2 Hearts.
2 Hearts Shows at leas five Spades; asking partner to bid 2 Spades.
With this information the open will set the suit and level for the contract.