Min pressure - min possible pressure used by painting engine (Paint tool, Pencil tool etc).. For example let’s wssume you set this to 50%. Now if you just slightly touch ipad screen with Pencil, painting engine will receive information that you used pressure 50%. It means you cannot paint very thin strokes.
Force for min pressure - lets say you set this to 20% - now if you tap screen with force < 20%, painting engine will receive info that you used min pressure (Min Pressure value described above).
In most cases both values should be set to 0.