These 'foam seals' are small, stryofoam O-rings made to fit between the DeWalt motors and their gearboxes. They form a seal against dust particles and other bad things you don't want in your gearbox. Think of it as a $1.00 insurance plan.

Need some extra foam o-rings for the DeWalt 12V, 14.4V or 18V motors? Grab some here.