Quickest way to find out is to try to wipe it off with some warm soapy water and a rag. It's probable that someone waxed the floor or used one of the many floor cleaner/polishes that deposit stuff, and this is just build up.

I'm guessing these are engineered or laminate floors with a cheap top layer, and what you're seeing is the top layer worn off. But obviously hard to tell in pics.

Dirty floor, time for a deep clean.

thank you all for the advice :)