Speed washers are thin, metal washers that are slipped over the axle between the nut and the bearing, and the hanger and the bearing. Though they go by many different names (speed washers, speed rings, bearing washers), all speed washers have the same purpose: to reduce friction in order to help the wheel turn faster. They serve as a buffer between the nut and the bearing, which allows your wheels to rotate quickly while also protecting the bearing face from damage.
Speed washers are an optional addition, though any rider planning to skateboard at high speeds will definitely want to invest in them. You can replace them as often as you see fit–it really depends on how much and how hard you skate. When they start to appear worn down, or if you see any rust or corrosion, simply remove your nut and slide on new speed washers. Speed washer size is based on axle diameter, and since the vast majority of truck axles are 8mm in diameter, that is most likely the size you will need. If you purchase complete trucks, many sets include a speed washer for each wheel, though you can also buy them separately.
Info from: https://www.warehouseskateboards.com