I was playing chess this afternoon on my mac, as human vs computer. In the position below:
[FEN "6k1/5pp1/2p3bp/pqP1P3/2R3P1/PPQ5/4K3/8 w - - 0 24"]
I tried to place the Rook in front of Black's Queen (b4), but this action was blocked and the Rook remained in the same place. Later I completed the game by making another move.
Why is this so? Why can't I place the Rook in front of Black's Queen? Please explain this.