"Toe" angle is the angle at which the wheels of your RC car point inward (positive toe) or outward (negative toe) when viewed from above. See our article on toe for the basics.
Active toe describes a change in the toe angle as the car's suspension is compressed. For most cars, there is no active toe, that is, the wheels keep essentially the same toe angle as the suspension is compressed.
Some on-road cars are designed with an active toe adjustment, allowing you to alter how much the toe changes as you compress the suspension. This is similar to the bump steer setting which changes the toe angle as the suspension is compressed in the front of the car.
To learn more about adjusting toe and its affects on your car's handling, see our article on angle.