Rebound and Compression
Rebound is when the piston rod is extended or pulled out of the damper tube.
Compression is when the piston rod is compressed or pushed into the damper tube.
Each of these movements is controlled by a set of valving disks or plates on the working piston. When the piston rod cycles in and out of the shock tube, the valve disks displace oil within the shock and create resistance. The graphics below demonstrate this process in a mono-tube 
