The pin
The piece that cannot move, because something dearer stands behind it.
A pin is when a piece cannot move because something more valuable stands behind it on the same line. If the thing behind is the KING, the pinned piece cannot move at all — that is a rule, not a choice. Look at the marks: the knight on c6 and the king on e8 are on the same diagonal. All that is missing is one of your pieces attacking along it.
Pin the black knight: put your bishop on the diagonal that runs through c6 and ends on the king on e8.
This is the pinned knight, and it is Black's turn. Look at the board: NO square is marked. On paper the knight on c6 has several moves, and not one of them is legal — every single one would leave the king in check. That is exactly what a pin does: it turns a piece into stone.
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