Answer:
washers are the small metal rings that go between the bearing and axle, and the bearing and nut.
here is an exert from http://www.omnisk8.com.au/factory_bearings.htm
Make sure to use "speed washers" and "spacers". Speed washers give a slight gap in between your truck hanger on the inside and your axle nut on the outside. Spacers help to retain the two bearings as an integral unit, it spreads the sideways loads of skating between the two bearings and greatly reduce the chances of your bearings popping from either axial loads or over tightening your axle nuts putting pressure on the internal and external bearing rings.