This is just kinda a simple version, but if you're not drinking enough water, your cells will hold on to it (thinking that they need every little bit they can get). If you're drinking enough water, your body has no need to hold on to it, b/c you're letting your cells hydrate themselves enough. If you consistently drink enough water, your body recognizes that it's hydrated and let's go of all the excess water your cells have been holding on to.