Really quick and dirty. A simple shock strut design consists of 2 major parts: the inner cylinder and the outer cylinder. The outer cylinder is what you see of the landing gear and is fixed. The inner cylinder is the chrome. Attached to the inner cylinder are the bogies or axle depended on aircraft. This is the part that moves.
, so far? The shock strut is filled with a hydraulic fluid (Mil 5606 or equivalent is the most common), then an air charge is added. The air pressure introduced is depended on aircraft weight at the time of servicing. A simple chart is usually attached to the gear or nearby.
Inside the shock strut, attached to the inner cylinder is a tapered metering pin. This pin travels up and down with the cylinder. In the outer cylinder is a metering plate. Its fixed. When the aircraft lands the upward movement of the inner cylinder forces the fluid to pass through the metering plate. As the strut further compresses the pin enters the metering hole and begins to restrict the fluid flow. This snubs or absorbs the energy being exerted. The goal is to have the full eight of the aircraft on the gear before the pin (strut) bottoms out.
The air charge doesn't really have a role in landing except to maintain a head on the fluid (give it a little resistance). The aircraft exists to cushion the aircraft during taxiing. The aircraft does not exert enough pressure during taxiing to really bring the fluid into play. That's why the ride is so bumpy. Its only an air cushion.
Don't get me wrong, an air charge is required for a smooth landing, but it is the fluid charge that takes the brunt of the landing forces. I've taxies plenty of aircraft taxi with very low fluid and a good air charge. The ride is no different. (usually taxiing to hangar for repair of a leaking strut).
Again, that was quick and dirty. Numerous other components are installed within the cylinders to help the drivers make a smooth landing.