Changing volume spacers in FOX FLOAT NA2 forks is an easy internal adjustment that allows you to change the amount of mid stroke and bottom out resistance.
If you have set your sag correctly and are using full travel (bottoming out) too easily, then you could install one or more spacers to increase bottom out resistance.
If you have set your sag correctly and are not using full travel, then you could remove one or more spacers to decrease bottom out resistance.
Find your owners manual to work out the maximum volume spacers you can install in your fork:
FOX Owner Manuals