My college biophysics professor explained it to us this way. You have a large glass container, with an air proof seal. Sealed inside are two open beakers, one with 100 ml of a 1 M solution of NaCl and another with 100 ml of a 2 M solution. What happens over time, as water evaporates and condenses? Eventually, he gradually coxed the conclusion out of us, that the two vessels would reach an equilibrium with the same conc. of NaCl, and by extension, less water in the first and more water in the second. The semi-permeable membrane just allows them to be in contact while that happens.