even though there isn't an immediate checkmate, the Black player has all the space. White's K & Q are all locked down, giving him a completely lost position.
Here's what might happen if White defends differently after ...Qxe4 in greenpawn34's line.
FEN
6k1/5pp1/1r1p4/4pP2/P3P2P/1pP1R1P1/1P1R4/Q1K2q2 w - - - -
[fen "6k1/5pp1/1r1p4/4pP2/P3P2P/1pP1R1P1/1P1R4/Q1K2q2 w - - - -"]1.Rd1 Qf2 2.Rd2 Qxe3 3.a5 Qe1+ 4.Rd1 Qxe4 5.Qb1 {Intending to come next to d3.} Qe3+ 6.Rd2 Ra6 7.Qd3 Rxa5 {If now Qxe3, then ...Ra1 mate.} 8.Kb1 Qa7 {White has no good defense to ...Ra1+.}
In the position where White resigned, I don't see a good defense to Black's plan of capturing the a-pawn and bringing a major piece to a1.
