It is the ratio of the shock absorber resistance to Jounce (upward movement) and to rebound (downward movement). For example 50/50 ratio would mean equal resistance to Jounce and rebound. Most shock absorbers are design for 50/50 ratio only. It should be noted that this ratio only describes what percentage of the shock absorbers total resistance is in compression and what percentage is in extension. The actual damping rates are controlled by the size of the piston and orifices and closing force of the valves.