Bushings are rubbery rings that are fit into skateboard trucks. Skateboard trucks usually have two bushings per truck. One bushing is in the heart of the truck, and provides the flex and springiness of the truck, for turning and what not. The other bushing is smaller, and set right beneath the head of the kingpin, allowing the trucks to be tightened or loosened easier, and to a more specific degree. Some skaters like tighter trucks, usually for more tricks, while some skaters like looser trucks, often for cruising. However, tight or loose trucks has more to do with simply what feels right to the skater than anything else.
Bushings need to be replaced in your skateboard trucks every once in a while, because they break down and stop giving good feedback and springiness.
The bushings cushion the truck when it turns. The stiffer the bushings, the more resistant the skateboard is to turning. The softer the bushings, the easier it is to turn.Popular Bushigns are Bones or Khiro and they sell a range from Hard to Soft. Soft being around 85A and Hard being around 95A, but Brands like Khiro sells a MUCH larger range going to extra soft/hard.