1. White reroutes the knight to control g5
2. White provokes a dilemma
Prevention. A prophylactic move or idea is one that anticipates and defuses the opponent's plans/threats.
In position 1, white retreats their knight from e2 to g1 for it to re-emerge on f3, thus controlling g5. Controlling g5 is necessary because black, at a space disadvantage, intends to relieve their cramped position by playing ...Ne8 and ...Bg5, exchanging off his dark square bishop and giving himself more room to manoeuvre behind his pawns. With a white knight on f3, this is no longer possible.
In position 2, white has played 21. Be3-c1!! asking black to resolve the tension between the pawns on e4 and f5. The white knight threatens to go to the vacated e3 square and attack f5 in the event of black allowing white to take on f5. But a capture on e4 by black would give him isolated doubled pawns on the e-file. Had white played a different move to 21. Bc1, then black would have played 21... Bf6, hoping to entice white to play a potentially weakening pawn move on the queenside. After Bc1 the b2 pawn is protected and there is little chance of white reflexively weakening his pawns with b3.
Note: if black plays 21...Rd4, then 22. b3! leaves the rook without any good option but to retreat.