Spacers allow you to fit a shorter motor into a longer case. They are the size of 1 propellant grain. So, for example, you can fit a 3 grain motor into a 4 grain case using 1 spacer. Or, a 3 grain motor into a 5 grain case using 2 spacers. Do not use more than 2 spacers, because it can cause issues with the ejection charge.
ONLY use the standard spacer, unless you are using a 6XL grain case.