Anaerobic Threshold is the point at which there is a ‘sudden and sustained’ increase in blood lactate concentration.
Accumulation of lactate is associated with the onset of fatigue; therefore the higher the intensity this occurs at, the greater resistance you have to fatigue. This is the highest possible steady-state work intensity that can be maintained for a prolonged period of time (so strongly related to FTP and CP).
Above this intensity you would begin to fatigue more rapidly. Like the lactate threshold, this can be improved with training. An increase in the exercise intensity at which the anaerobic threshold occurs could indicate an improvement in lactate buffering or lactate clearance.