Reverse osmosis separates water from the substances dissolved in it, using a partially permeable membrane — a barrier with pores so tiny that water particles can pass through, but larger dissolved particles (like salt) cannot.
Normally, water tends to flow through such a membrane towards the saltier side (that natural flow is called osmosis). In reverse osmosis we apply pressure to the salty side, forcing the water to flow the opposite way — out of the seawater and through the membrane — leaving the salt behind.
Because the applied pressure pushes water in the reverse direction of its natural osmotic flow. No heating is needed, unlike distillation — the separation is done by pressure and a membrane.
Mixtures of a liquid and substances dissolved in it — like salt in seawater — where filtration would not work (dissolved particles pass straight through filter paper). The membrane's pores are far smaller than a filter paper's, so only water gets through.