Spacers are found in-between the two bearings in the centre of your wheel.  Without spacers, your wheels won't roll properly.

You need one spacer for each wheel on your skates.

Most inline skates use 8mm bearing spacers.

These will not work with skates that use 6mm axles.

Sold in a pack of 4, 6, 8, and 10.

Standard spacers

Here's what a standard wheel and spacer look like.  The core has a large opening which will accommodate the "standard" bearing spacers.

Slim Spacers

Smaller sized wheels (60mm and below) from Undercover and Dead usually use a core with a smaller opening (as seen in the photo below) and require a "slim" spacer.