Sold as a set of 8 speed washers.

These are the small little rings that come on most skate axles when you first buy trucks.  Although not a critical piece, these do allow for some fine tuning in several situations.

Most frequently these are used with classic bearings and bearing spacers to get just the right amount of snugness to your bearings.  

In a few cases they may be used to keep the face of your bearings on the inside of the hangar or axle nut.

Some people even use them when they need just a bit more clearance.