The Jacoby convention is designed to explore a slam in a major suit.
If the responder has at least 4 card support in the openers major suit and at least 13 points she bids 2NT.
The opener must rebid as follows -
3 of the major showing at least 17 points
4 of the major showing less than 17 points
3 of a new suit showing at least 17 points and a singleton ace or void in the new suit
The responder can now decide on the course of action, either explore a slam or close the bidding at game level.
NB Jacoby is not for use with minor suits. A response of 2NT to an opening bid in a minor shows no four card major and 10 to 12 points.