The point is that after
1...Rxe3 Black will be winning material, as the sole defender of the white knight has been removed, and a pin tactic is coming. Specifically, after
2.Rxe3 (anything other than this recapture means Black has just won a piece for nothing) and
2...Bxd4, White has no way to stop the follow-up
3...Bxe3+ since the rook is pinned to the king, and so Black has netted an extra piece anyway.
[FEN "r1bqr1k1/1p3nbp/2pp2p1/2n2P2/p1PN1P2/2N1B1PP/PPQ3B1/R3R1K1 b - - 0 1"]
1...Rxe3 2.Rxe3 Bxd4 *