A spacer is an aluminium tube that has to be installed between the bearings (inside your wheels) to prevent from jamming inside the bearing.
There are 6mm and 8mm spacers. Depending on the axles of your frame you have to insert the correct spacer. 6mm spacers reduce the internal diameter of the ball bearings from 8 to 6mm and are as wide as your wheel and ends on the outer edge of the ball bearing (cf. right picture blue 6mm spacer). 8mm spacers will be positioned directly between the ball bearings and are virtually invisible from the outside.
We offer overlong 8mm spacers (width 10,3mm) to compensate production tolerances and to prevant jamming.