Osmosis occurs because dissolved particles have a higher concentration on one side of a membrane, that only allows the passage of water, than the other. As water molecules randomly drift across the membrane, they tend to stay around the dissolved particles rather than drifting back.
Osmosis exerts pressure and can cause the amounts of water on either side of a membrane to be very different, despite the fact that water can flow freely across the membrane. Osmotic pressure continues until the ratio of dissolved particles to water molecules is the same on both sides of the membrane. This property of osmosis makes it very important for living organisms. The transfer of water from place to place in an organism can be managed through osmotic pressure, which is essential for many biological processes.
Osmosis is driven by the thermal energy of the water on the side of the membrane with purer water. This is what causes the water molecules to move around and occasionally cross to the side with more dissolved particles. The pressure exerted by this movement can be relatively high. Pure water placed next to human blood across a membrane would flow into the blood side strongly enough that it would take seven times the pressure the atmosphere exerts at sea level to stop the flow.