The beat is the vibration (half-oscillation, or semi-oscillation, or “tick” of a movement. Hence, the beat error is the difference between the clockwise swing and the counterclockwise swing of the balance. In perfect conditions, the beat error is 0.
The balance wheel swings clockwise and counterclockwise. Each one of those swings, in either direction, is called a “beat.” If the watch runs perfectly, the clockwise swing will be the same as the counterclockwise swing, and both swings will take the same amount of time.
If the clockwise swing takes a bit longer than the counterclockwise swing, the watch is slightly “out of beat.”