Stayman

Stayman is a convention (not natural) that is mainly used when partner opens 1NT (12-14).

Bid 2C to ask whether Opener has a 4 Card Major.

If 1NT Opener has a 4 Card Major then she bids that Major at 2 level - 2H or 2S.

If she has BOTH 4 Spades and 4 Hearts, then bids she 2H.

If she has no 4 Card Major, then she bids 2D.

Responder must have a plan for whichever answer is given and bid accordingly after Opener replies.

Opener must not bid again.

You should really have 11 points to bid 2C Stayman. If you have both a 5 Card and a 4 Card Major  you can bid 2C with fewer points.

If you have 13 points and a flat 4, 3, 3, 3 shape, better to jump to 3NT even if the 4 is a Major.

If opposition Overcalls or Doubles the Opening Bid  then the Stayman convention is off and all bids are natural.

IMPROVERS 

Stayman is also used after 2NT (20-22) Opening. Bid 3C (Stayman, asking for Majors).

Stayman can also be used after 1NT OVERCALL (16-18) if agreed with partner.

1D - 1NT - PASS - 2C (asking for a 4 Card Major)