Put fresh water and salt water on either side of a membrane that passes water but not salt, and water flows toward the salt. Push on the salt side harder than its osmotic pressure and the flow reverses — that is reverse osmosis.
The pressure needed is set by the osmotic pressure difference: roughly 25–28 bar for seawater, and more as the brine concentrates along the vessel. Plants therefore run at 55–70 bar and recover about half the feed.