What is a bushing?
Bushings are the rubber (actually polyurethane) bits around the kingpin. They provide the rebound of the truck, the harder the bushing, the harder it is to turn the truck.
If you are a lightweight you should have a softer bushing. And if you are a bit larger or want to go downhill you should have a harder bushing. Nowadays there are so many different types of bushings that it's hard to say which ones are best for you, our advice is to start with the stock bushings and start experimenting from there.
Just like wheels, the hardness of bushings is measured in durometer (a) and they come in various different shapes. It is also a fact, that your setup will feel more reactive at higher speed, that is why you can get harder bushings or more restrictive bushings for downhill and softer for cruising.
You will need two bushings per truck. The one closer to the board is on boardside, the one closer to the road is on roadside.