The Pin
A pin freezes an enemy piece in place: it cannot legally (or sensibly) move because doing so would expose a more valuable piece — or the king — directly behind it.
The rook on e2 pins the knight on e7 to the king on e8 — an "absolute pin". The knight cannot legally move.
Absolute vs. relative pins
An absolute pin is against the king — the pinned piece literally cannot move (it would be illegal). A relative pin is against a more valuable piece; moving the pinned piece is legal but loses material.
How to exploit a pin
Pile more attackers onto the pinned piece — it cannot run. Pawns are especially good at winning a pinned piece, since the piece can't move away from the attack.
Practice this pattern
Real positions where a the pin wins — tap a puzzle to solve it, or train the theme endlessly.