Chess - Dumb Horse

The worst piece: it can't move without taking an L

Joker reaction image about joke explaining incoming, but here I go.
In chess, Knights can only move in an "L" shape, in any direction (for instance, 2 spaces forward and 1 to the left, or 1 space backwards and 2 to the right, and so on). A knight stuck in the middle of a 3×3 board like this has literally no viable spaces to move to, thus the joke.