Shock pad
What is Shock pad in skateboarding?
The shock pad is a rubber or foam sheet that goes between the truck and the deck to absorb impact. Unlike a riser, it is not there for height.
Extended definition
A riser is hard plastic and lifts the deck to avoid wheelbite; a shock pad compresses and dampens the vibration and hits that reach the deck. It can cost you a bit of speed, so if all you need is height against wheelbite, a riser is the better pick.