Plain English Meaning
It's the steepest angle the wing can meet the oncoming air before the air stops flowing smoothly over the top and the wing stops producing enough lift to fly. Go past this angle, and the wing stalls.
Full Definition
The angle of attack at which the airflow over the wing's upper surface separates and the wing stalls, causing a sudden loss of lift regardless of airspeed, weight, or attitude.
Context Anchor
Seen in discussions of stalls, slow flight, and flap use. In the flap chapter, it matters because extending flaps changes the wing’s shape and affects how the wing behaves near a stall.