In Philidor's position the rook stands next to its king on the edge and Black is to move. Every rook move allows a queen fork or a mating check; this is the target position for the whole ending.
This is the key position of queen versus rook. Black's king is on b8, the rook on b7 shielding it, White's king on c6 and queen on a5. With Black to move, the rook must leave the b7 square (king moves are impossible or lose at once), and wherever it goes a queen check picks it up.