The shock pistons live inside the shock. Pistons have small holes, and as they move up and down, the oil within the shock is forced through those holes, slowing it down an creating a damping effect.
Some RC cars have a wide range of pistons available. Typical properties to watch for are:
Number of holes
More holes means less damping
Size of holes (or mix of sizes)
Smaller holes creates more damping and pack
Flat pistons travel at the same speed up and down, but tapered pistons travel faster in the direction of the taper - the taper is usually on the bottom so the shocks can extend more quickly.
Thin pistons produce less pack than thick ones.
Molded plastic pistons are cheaper but not as precise. Machined pistons are more precise, so their damping is more consistent and smoother.
See our damping article for details on how to tune your car's handling using shock pistons and oil.