This is a forcing sequence after 1x - 1y - 1NT, were x and y is a symbol for a natural bid at the 1-level. The advantage for such a convention is that it make it easier to make a forcing bid without jumping in a new suit you may not have.

-2C: Demands 2D. (To play 2 diamond / limit)
                -2H : Limit.
                -2S : Limit.
                -2NT : Limit. Denies 3433/4333
                -3C : Limit. 
        -2D : Art. game forcing.
Further bids are natural. Bid as low as possible. If pd has bid 1S you show your 4
card heart suit before showing 3 card spade suit. If the 2D bider bids 3 i major this 
shows a slam invitation.
-2H : To play if: 1min - 1H - 1NT - 2H. / Ask for support in for of the suits.
-2S : To play if: 1min - 1S - 1NT - 2S. 
-2NT : Limit. 
-3C : To play. Shows 6 card club suit & kort major.
-3D : Weak limit. Shows 6 card diamond suit & 4 card major.