Cell membranes are particaially permable. Water molecules can get through their smaller holes. Overall, more water molecules diffuce through partially permeable membranes from a diluted solution (with many water molecules) to a more concentrated solution (with fewer water molecules) This is called osmosis.
Liquid and gas molecules move randomly all the time. Overall, they move from an area of high concentration (where there are many of them) to an area of low concentration (where there are fewer of them). This is diffusion. It is a passive process- it does not need energy.
Sometimes a cell needs to take in molecules (such as glucose) that have a higher concentration inside the cell than outside it. The cell uses active transport to do this. Repiration provides energy for active transport.