Some small molecules can readily diffuse through the lipid bilayer of the cell membrane or through the protein channels across it. Glucose and other large molecules cannot do this and membrane proteins must actively pump these molecules across the membrane. Because the membrane is selectively permeable, concentrations of dissolved substances, or solutes, can build up on one side of the membrane and not on the other. The resulting solute concentration gradients affect the movement of water into and out of cells.
Water by itself cannot get more concentrated or less concentrated (the hydrogen bonds between the water molecules stop them from getting closer together or drifting apart). Water only becomes "less concentrated" when solutes are dissolved in it. Another way of saying this is that a water concentration gradient is influenced by the number of molecules of all solutes that are present on both sides of the membrane.
The direction in which water moves across a membrane is influenced by tonicity, the relative concentrations of solutes in the two fluids. Water tends to move to where the solute concentrations are greatest. When a membrane separates two isotonic fluids, solute concentrations are equal on both sides of the membrane and there is no net movement of water in either direction. When solute concentrations are not equal, water tends to move from the hypotonic solution (less solutes) to the hypertonic solution (more solutes). The name given to this movement of water across membranes in response to solute concentration gradients is called osmosis.