Explanation

In the "Fundamentals" course you learned how to defend an attacked piece by running away to a safe square, blocking the attack, or protecting the piece. In the "Defence" course, several other methods can be learned, including defending by check or capture, and counter attacks.

But sometimes a defense is not possible, or is only possible with material losses, and the attack on the piece immediately wins material. When that happens, the piece is trapped. Here is an example:

The black knight on h1 has ventured too far into the enemy camp and has no retreat! A knight in the corner can only go to two squares. White already controls f2 and g3 and can trap the knight with 1.Kg1 (or even 1.Ke2).

The black bishop on e6 is blocked in by its own pieces (Nd7, f7), and has no safe square, because d5, c4, b3, a2, f5, g4, and h3 are all covered by White. The bishop can be trapped with 1.d5.

The black queen on b6 has only one safe square, a5, but White can take that square away and trap the queen with 1.a5.

Examples

White can trap Black's bishop d6 with 1.c5.
The bishop has no safe square to escape to.

White's queen has advanced deep into enemy territory, but it has run out of squares.
Black can trap the queen with 1...Ne6!

Sometimes pieces can get trapped even in the endgame:

Black only has a king left against bishop and knight but can trap the knight in the corner to achieve a draw with 1...Kb7.

How to spot the opportunity to trap a piece

If an opposing piece has no or very few safe squares to go to, always be alert for ways to attack and trap it.