This is a video on a trap against the bishop opening ( for black )
1. e4 e5
2. Bc4 Nf6
3. Nc3 Ng4!
4. Qxg4 d5 (discovered double attack)
5. exd5?? Bxg4
5. Qd1 dxc4
In the above two pictures the circled knight cannot move as this will make black suffer the loss of his queen and it is the same when the black king is in the situation like of the black queen . Now we say that the knight is pinned to the queen