Spacers are found in-between the two bearings in the centre of your wheel. Without spacers, your wheels won't roll properly.
Sold in a 4 pack.
You need one spacer for each wheel on your skates.
Most inline skates use 8mm bearing spacers.
These will not work with K2 and other brands that use 6mm axles.
Some wheels have smaller cores and will only accept the 8mm "slim" bearing spacers.
Brands like Undercover, Street Artist, Gawds, Dead will only take the "slim" bearing spacers.