Stemming directly from the work of Leo Baron, the 2NT response to an opening suit bid at the one-level is game forcing, showing 16-18 points and a balanced hand, although modern practice is for simply 16+ points and a balanced hand. Normally, both opener and responder continue by bidding four-card (or longer) suits upwards until they locate a fit or they reach 3NT.
For example:
It is still quite common to play 2NT over 1♣ or 1♦ as Baron but, increasingly, players are using 2NT over 1♥ or 1♠ as a game-forcing raise of opener’s major. Some people play that 2NT promises at least three-card support for opener’s minor.