Smaller than stuff are fields. Discrete bumps in fields are stuff. Fields are everywhere. Fields and their bumps interact with one another. There was a problem in completing the description of the observed universe based on fields that lead to the supposition of the higgs field. The higgs field interacts with other fields and their bumps to yield mass.

I am not a physicist nor have I studied or even worked with these descriptions of the smaller than small, but that's essentially it.