Why can't I place the rook in front of the queen? 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.
The Rook on c4 is pinned against the your King by the black Queen. Thus you can't move it without exposing your King to the Queen's wrath.