？
Did you know that you lose skin every day?
Literal
Every-day skin [subject-が] disappears [quotative-って] knew?
って is the colloquial form of the quotative particle と, ubiquitous in casual speech. 知ってた is a contracted past of 知っている — asking 'did you already know?' 皮膚がなくなる literally means 'skin disappears,' a somewhat blunt way to describe the daily shedding of skin cells.