Bushings are essential for the mobility (steering) of your board.
Bushings are small rubber buffers that are built into the longboard or skateboard truck. Different hardnesses allow different steering properties to be achieved for just as different driving characteristics and driver weights.
You can control the steering of your longboard truck via bushings. The small bushings are available in different hardnesses and the harder the lek rubber, the more stable the truck is, e.g. at higher speeds.