Anti-squat angle is the angle at which the rear suspension pins are tilted upward at the front when viewed from the side of the car. 0 degree anti-squat indicates the pins are parallel with the chassis. If the front of the pin is lower, it is "pro-squat."
In the front of the car, the suspension pin angle is usually referred to as kick-up, or sometimes "pro-dive" (more common in on-road.) If the pins are angled downward in the front, it's referred to as anti-dive.