A shock dyno is a machine that strokes a shock at controlled speeds and measures the force it produces, plotting compression and rebound damping against shaft velocity so a damper can be verified, matched or compared over time.
PRI describes it as equipment that records the force needed to overcome a shock’s resistance to compression and extension at a given velocity and stroke length. The chart you get back is force versus velocity, compression on one side of the line and rebound on the other. Penske splits the useful shaft-speed windows into 0–2 in/sec for body movement in low to mid-speed cornering, 2–6 in/sec for braking and direction changes, and above 6 in/sec for bumps and curbs. Racers dyno to confirm a rebuild came back on spec, to match a left-to-right pair, and to catch fade from worn seals, lost gas pressure or contaminated oil before it shows up as a handling change nobody can explain.
A dyno sheet turns "the car just feels different" into a number you can compare to the last one, which is the only way to tell a tired shock from a bad setup.