Pin
A pin attacks a piece that cannot move — or cannot move without unacceptable loss — because a more valuable target stands behind it on the same line. When the rear target is the king, the pin is absolute and the pinned piece has no legal moves at all. A pinned piece still attacks: knights and bishops remain dangerous even while tied down.
The bishop on b5 pins the knight on c6 against the king on e8 — moving the knight is simply illegal.