This is the main line of the Winawer Variation. Black has just pushed forward his c pawn with 4...c5, threatening white's pawn chain. White's knight on c3 prevents him from extending the chain by moving a pawn to c3, so this puts white's center pawns in danger.
However, c5 also blocks the escape route for black's dark squared bishop. White usually takes advantage of this to regain the initiative with 5.a3. Black usually trades bishop for knight with 5...Bxc3+ 6.bxc3.