Osmosis

What happens is that the solvent, water, passes both ways through the semi permeable membrane. The stronger solution, containing more molecules of the solute which are too big to pass through, blocks more of the water molecules from going through, whereas the weaker solution lets more water molecules through. Eventually the weaker solution becomes more concentrated, while diluting the stronger solution until they become equalised. Think of it as a box of footballs and tennis balls divided by a barrier with holes that will allow the tennis balls through, but not the not the footballs.