Once you control an open file with a rook, you can often use the rook to penetrate your opponent's position. In the following diagram, the position looks quite even, but it is white's move. White takes the opportunity first to move the rook to the open file. Black cannot safely counter by moving his rook to the same file because white would immediately capture it.
It is therefore not possible to challenge the white rook, which can show off its strength by moving to the seventh rank in order to attack the black pawns. Once there, the white rook attacks two pawns simultaneously. Try as he might, black can only defend one of them. In positions like this, we say that white has a good rook while black has a bad one. It sounds almost simplistic, but masters always try to make sure that their pieces are better than their opponent's pieces.