are the little rubbery rings inside your trucks. They wear out and can lose their spring, and your board will end up always turning in a certain direction, or simply losing performance. Bushings need to be replaced a lot sooner than most skaters think. The nice thing is, skateboard bushings are easy to replace.
Before you replace your skateboard bushings, I recommend removing your skateboard's trucks. This isn't necessary, but it will make things a lot easier. Read How To Remove Your Skateboard Trucks.
Now that you are ready, check and make sure that you have the right tools, and understand what parts we're working with...