White sees that Black has left two pieces—his bishop and his b6 knight—a square apart on the same rank, inviting a pawn fork. White’s c-pawn needs two moves to get there, which is fine so long as the first move is a threat that requires a time-consuming reply: after c2-c4, the knight moves, and then c4-c5 wins a piece by forking the knight and bishop. But wait; where is the cover the pawn needs before it can attack a bishop that has the power to capture it back? There is none, but none is needed because after the knight on d5 moves the bishop is pinned to its king by White’s rook on d1. Recall that even if a target looks unsuitable because it can defend itself, it is powerless to do so if it is pinned; thus whenever a piece moves, as the d5 knight does here, consider whether the move creates any open lines, any pins, etc. (If Black were to reply to the initial pawn push with Nb6xc4, he would be leaving his other knight exposed to RxN; another question to ask when you imagine a piece moving is what it used to protect that may now be loose.)
Now let’s think about counterplay for a moment. After c2-c4, is it clear that Black has nothing better to do than move his knight out of the way and allow the fork on c5? What attacks of his own does Black have as options? He can play BxB, freeing one of the pieces to the potential fork with a capture; the question is whether, after White recaptures h2xB, Black has bought time to rush his knights to safety. Almost, but not quite. For when Black played BxB, the knight on d5 became pinned to its king and thus cannot now be moved. So after White recaptures on g3, the best Black can do is take the c4 pawn with his other knight (Nb6xc4), thus allowing White to play RxNd5 next move. White wins a piece for a pawn, but again the sequence is not quite as simple as it might look at first.