| A discovered attack is a tatical theme in chess. It occurs when a rook, bishop or queen attacks an enemy piece, but a friendly piece blocks the attack. Discoveries are most useful when they are a part of a double attack, or when the friendly piece moving out of the way poses a threat as well.
Here is an example game containing a discovered attack.
1. e4 e5
2. Nf3 Nc6
3. Bc4 Bc5
4. c3 Nf6
5. d4 exd4
6. cxd4 Bb6
(This is the Giuoco Piano opening)
7. e5 Ng4
8. Bxf7+ Kxf7
Right now white's queen is indirectly attacking black's knight. White's knight is in the way.
9. Ng5+
As the white knight moves, the queen's attack on black's knight is discovered. The white knight also attacks black's king. Black cannot protect his king and his knight in one move.
9. ... K moves
10. Qxg4
And white makes up for his sacrifice. | 
