A discovered check is much like a discovered attack, with one difference. The victim of your discovered attack is your opponent's King. In the diagram on the left, White is about to play Bd3+. The check doesn't come from the Bishop, however. It comes from the unmasked Rook behind it.
Black has to get out of check, and then White can take the Queen with Bxg6. Although Black can block the check with his Queen, after Qe6 comes Rxe6, and White would win 4 points.