Spacer is the distance between your ball bearings inside the wheel. Like so much else in skate, the standard dimensions of 1cm are usually used. Note that the tolerance between different wheel manufacturers may differ slightly by one hundredths (which means that if you use the same ball bearing and spacer on one wheel, you may be able to tighten your nut to the maximum without the wheel locking, while on another brand of wheels you may not can fully tighten the nut without affecting the wheel roll).
It works by making sure that your ball bearings are not strangely compressed if you tighten your shaft nut. You can still ride without it, but if you want to make sure that the ball bearing tracks do not become skewed or brake your wheel when you turn, it is recommended to use this. A small cost to improve. Some ball bearings already have this built in, this type of ball bearing is usually called built-in.
We recommend that you always use spacers in your ball bearings as it actually improves performance. In other words, it can be quite a waste to buy expensive ball bearings if you ignore using this little washer because then you can not enjoy the best performance from them. But you can also choose to use ball bearings that have it built in as I said.