A pin occurs when an attacked piece moving would expose a piece

behind it to danger of being captured

In the following animation,the white bishop pins the black queen

to the black king,the queen can't move to safety because the king

would be in check,so next move,the bishop captures the queen..

If the queen were to capture the bishop, the white knight would

capture the queen...so...a bishop for a queen,NOT A BAD TRADE !!!

This is an example shows the power of a pin.