This is a convention who is used after strong NT. Either 2NT opening or 2C-2D-2NT, where 2NT is strong balanced. The good thing about this convention is that you can have a 5 card major suit and a balanced hand when you bid strong 2 NT.

3D : Have at least one 4 card major suit.
3H : 5 Hearts. 
3S : 5 Spades.
3NT : Denies 4/5 card major.

3D- 3H: 4 Spades. not 4 Hearts.
3S: 4 Hearts. not 4 Spades.
3NT: To play.
4C: 4-4 in major. Slam invitational.
4D: 4-4 in major.