Minor-suit Stayman is a convention after a NT opening, indicating at least invitational strength and at least 5-4 in the minors.
In most partnerships, to use MSS over 1NT, bidding 2♠ initiates the convention. Opener's rebid are as follows:
Responder then may pass, correct to the other minor, return to NT or bid game.
A third possibility of bidding hands with the minors starts by 1NT - 2NT, where 2NT is treated as a transfer to Diamonds but can also hold a really weak 5-5 in the minors hand. Opener bids 3♦ if (s)he likes Diamonds, and 3♣ if (s)he doesn't. Responder passes either bid with the minors, and corrects to 3♦ if Opener happens to bid 3♣. Then, 1NT - 3♣ is Invitational MSS, and 1NT - 3♦ is MSS, but Forcing to Game or higher.
It may also be used over natural 2NT or 3NT openings, but come up rarely over 2NT and are non-existant over 3NT due to many people using 3NT for something else, and the minute probability of holding a balanced hand of 25-27 HCP, let alone being able to open it 3NT.