Black recently moved his bishop to a3; then White took it with his b-pawn. Now it’s Black’s turn again, and time to figure out why he let his bishop get taken. Look for loose White pieces and you come to his rooks—and see that the diagonals leading to them now are open on both sides. Black can't attack them both at once, and he has no checks or useful captures to consider. But it's important also to consider threats, since the replies to them tend also to be forced; they let you hold the initiative and control the play. Here you ask which Black pieces can attack either White rook. There is only one answer: Qd4. It doesn't win anything by itself, but ask anyway what will White play in response. Suppose he moves the rook to b1. Now ask the same questions: you still are trying to make something out of the two loose Black pieces—and now you can attack them simultaneously, with Qe4. (White’s better reply to Qd4 thus is Bb2, interposing his bishop; it gets taken next move—for now it, too, has been left loose—but then White at least has a move to save his rook.)
This position is a good example of a point made earlier: a rook frequently is unprotected when it sits in the corner of the board, and so is vulnerable to a fork if its diagonal pawn cover is blown. This is an unusual case where both rooks are in that position. Think about it if you or your opponent moves the pawns at b2, b7, g2, or g7 early in the game.