The remainder of the wash that is left over in the Air Still or Air Still Pro boiler after distillation, often referred to as 'backset', can either be discarded, as it is mostly water, or can be reused in subsequent distillation runs.
Reusing backset means that you may increase your yield slightly, as there is always a small amount of residual alcohol in the 'backset' that cannot be captured during distilling.
If you want to reincorporate your backset with your next wash, be sure to cut the ABV of the backset to under 40% ABV before adding it back to your Air Still or Air Still Pro and distilling again.