Osmosis is the reason a raisin plumps and a lettuce leaf wilts. Two solutions of different concentration are separated by a membrane that lets water through but not the solute, and water moves down its own concentration gradient until the pressure it builds up stops it. You can do it in a beaker, in a cell, or in a potato.
The solute — the salt — moves by diffusion, driven by the random thermal motion of the molecules, and it goes from the concentrated side to the dilute side until the concentrations match. That is ordinary diffusion, and it needs no energy at all.
The water moves by osmosis, and the direction is the opposite one: it goes towards the concentrated side, because the solute there occupies space and lowers the water's chemical potential. Water keeps moving until the pressure it has built up on the concentrated side exactly balances the difference, and then everything stops.
The equilibrium is a balance of pressures, and it can be written down: the osmotic pressure difference is π = RTΔc, and at body temperature a one-molar difference in an ideal solution gives about twenty-five atmospheres. That is why a cell membrane has to be built so carefully, and why the kidneys spend a great deal of their effort moving salt against a gradient.
Here the solute is too big to cross the pores and the water is not, so only the water moves, and the left compartment swells until the pressures balance. Turn the pore size up and the solute starts to leak through, and the whole argument collapses into ordinary diffusion.