Ride height is the height between the body of the car and the ground and it is measured from the front and rear axles. The height is given while the car is stationary, and varies when the car is moving, due to downforce, wheel travel, pitch and roll.
Ride height affects the car behaviour by lowering the CG, making the car pitch and roll less.
Lowering the ride height also raises downforce. Although a lower ride height gives a better vehicle comportment, it is desirable for the car chasses to touch the ground on the bumped tracks or at high downforce values.