Water has high surface tension. Diffusion is the result of thermal motion. This is a kind of passive transport. If there is a membrane that won't let a solute pass, the solvent molecules will flow until the concentration is the same on both sides. The pressure needed to keep the volumes the same is the osmotic pressure.
Water flows from lower to higher osmotic pressure.
The movement of water across membranes and balance of water between the cell and its environment is crucial to organisms. Cells shrivel when in a hyperosmotic medium. Soaking your foot in epsom salts solution to reduce swelling. Eye drops are isotonic, many marine animals are isotonic with sea water. This is usually lethal.