A sliding-stem valve body is one where the moving parts slide with a linear motion. Some examples of sliding-stem valve body designs are shown here:
Sliding Stem Valves
Most sliding-stem control valves are direct acting, which means the valve opens up wider as the stem is drawn out of the body. Conversely, a direct-acting valve shuts off (closes) when the stem is pushed into the body.
Of course, a reverse-acting valve body would behave just the opposite: opening up as the stem is pushed in and closing off as the stem is drawn out.