Keyword Q&A
[ Bishop Pin Knight ]

In the following image, Black's dark-square Bishop is attacking White's Knight:

Ideally, White would move the Knight, but it can't ... it's protecting its King. To move would be to deliberately leave the King in "Check", which would be an illegal move.

So, White's Knight must stay where it is, "Pinned" by Black's Bishop.

Because the King is involved, this is known as an "Absolute Pin" - until White's King gets out of the potential attacking line, White's Knight must "Absolutely" remain, "Pinned" on its current square.

Had it not been the King, behind, but another Piece, of greater value than the Knight; that would be a "Relative Pin" - Relative, because White's Knight could move to escape capture ...

It would all depend on the risk/reward of placing the rearward Piece in jeopardy, something that is "Relative" to the unique position on the Board, at any given time.