I’ll help!! So Hydration refers to cells absorbing water. Our cells need water in order to remain healthy and humans need water in order to live. Keep remembering, water goes wherever water is lower. So the water can spread throughout wherever it’s needed. It can be stored inside the cells themselves, blood, intercellular fluids, etc. we can measure how much water has entered the cell by measuring the movement of water caused by diffusion ( a little harder to explain so I’m sorry if that doesn’t make sense!)