A Discovered check is really an uncovered check. A piece is moved so that the line is open so that another piece is attacking the opponents king. It is also possible to uncover an attack on your opponent's queen or on any of your opponent's men.
In the diagram on the right white can play Nc7+ and an attack on the rook is uncovered.
When black moves his king out of check the white bishop can capture his rook Bxg8.
Look at the diagram on the right. Is it safe for white to capture the black knight with his queen?
