Read the imbalance
Black's c- and e-pawns have advanced and can no longer control d5. White's c3-knight can occupy that square, and no black pawn can drive it away. The weak square becomes relevant because a piece can reach it immediately.
Connect the feature to a plan
The plan is not simply ‘use the weak square’. White wants to install the knight on d5 before Black reorganises. If Black exchanges it with ...Nxd5, exd5 leaves White with space and an open e-file.
Why the move is reusable
The lesson applies beyond this position: a permanent square has value only when the correct piece can occupy it and the resulting exchanges remain favourable.