footwear (shoes or slippers) with no heel (or a very low heel)
"After running errands, I slipped my feet into my comfortable flats to rest at home."
plural of flat
"The dancers performed their routine on the wooden flats in the back of the theater."
Third-person singular simple present indicative form of flat
"The river flats during heavy rains."
In plain English: To flats means to flatten something out so it is no longer curved or round.
"Don't forget to flatten the dough before rolling it out."