Lesson
White to move. Main line: 1. Bxe7
Bxe7 simply captures the queen, yet Black cannot reply Kxe7. Once the bishop occupies e7, the rook on e1 attacks that square up the open file; a king is never allowed to capture onto an attacked square.
This is the practical effect of an absolute pin. The queen is not literally unable to move before the capture, but the king behind it cannot function as a defender. White should evaluate the e7 queen as tactically loose.
When counting defenders, exclude any king capture that would step into check and any pinned piece that cannot legally move. A piece can look defended on a static board while having no legal recapture in the actual position.