A silly position I made up to demonstrate how pins work.

This chaotic (and fake) position is in fact checkmate for black, despite the fact that:
A) The rook is attacked two different ways,
B) The bishop that would be "attacking" the king if the rook was taken by the knight is in an absolute pin, and
C) The other bishop "attacking" the square the king would otherwise be able to move to is also in an absolute pin.

Notice also that the black queen is pinned, preventing it from taking the rook and saving the game. Notice also that the queen taking the rook on a3 would normally put white in check (that was honestly unintentional. I only realized it after I posted this), but that doesn't matter either.

The key takeaways here are as follows:
- PPDD - Pinned Pieces Don't Defend. (I stole that from someone.) If not for the rook on the back rank and the light-squared bishop, this position would be a win for black.
- Even though they don't defend, pinned pieces can still contribute to checkmate or otherwise restrict the king. This is because their king would be the one to get captured first; it's the same reason why checkmate is always a victory, even if your opponent could have checkmated you on their very next move.

The bottom line is that the person whose king will die first is the loser, no matter what. 

I hope this is helpful. It sure has contributed to quite a few wins (and losses!) in my games.