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 2 grain motor into a 3 grain case using 1 spacer. Or, a 2 grain motor into a 4 grain case using 2 spacers. Do not use more than 2 spacers.
 
Click here for instructions on how to use the spacers (info is on page 2).
ONLY use the standard spacer, unless you are using a 6XL grain case.