Applied pressure pushes water through a semipermeable membrane while salt ions are rejected. When applied pressure exceeds the feed's osmotic pressure, fresh water flows through; when it doesn't, osmosis pulls water the other way.
π = i·M·R·T (van 't Hoff). Net driving pressure ΔP − Δπ must be positive for fresh water to cross. Feed concentrates as permeate is drawn off; permeate carries a small salt leak set by the rejection rate.