How big is 15.3 hands and 17.1 hands?

My mom said my riding horse is 15.3 hands and my pasture ornament lame horse is 17.1 hands. I'm not sure how big a hand is and I was wondering how big one is and how many feet the horses would be? Sorry I know so little haha. I've lived in the city for all my life and then we moved to a big country house and my mom said now that we have the place and time for she's getting back into them. :D