Rebound is the damping force a shock makes as it extends — how hard it resists the spring pushing the suspension back out. Compression damping is the other half, controlling the shock as it collapses.
Rebound is where a lot of short-track shock tuning lives, because it controls how fast weight comes back off a corner. QA1 frames it as valving that “allows your car to absorb bumps during compression but keeps the shock from extending too easily during cornering.” Too little and the spring runs loose, the body rolls freely, and the car feels sloppy. Too much and the ride goes harsh, the suspension can’t follow the racetrack, and a tire can get skipped off the ground over bumps. Rebound is also how you hold weight on a corner longer, which is why teams put shocks on a dyno instead of guessing. RC racers reach the same behavior through shock oil weight and piston choice.
Rebound decides how quickly the car gets its weight back after a bump or a corner, which is the difference between a car that recovers and one that never settles.
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Race Car Shock Tuning: What the Valving Numbers Mean and Which Shock to Change