This is a membrane which has many small holes in it which allows the solvent to flow through but not the solute. This is known as osmotic pressure. A simple example can be seen when making a pickle. A cucumber is placed in a brine solution and the skin of the cucumber acts like a semipermeable membrane, allowing the water in the cucumber to flow out into the brine solution. You end up with a shriveled cucumber.
Osmotic pressures are often quite high.
When working with ionic solutes in water and other polar solvents, one must be careful to take into consideration the fact that the ion concentration is higher than the concentration of the solute.