Diffusion - the process by which molecules spread from areas of high concentratiion, to areas of low concentration. When the molecules are even throughout a space - it is called EQUILIBRIUM
Concentration gradient - a difference between concentrations in a space.
Watch this animation of water molecules moving across a selectively permeable membrane. Water molecules are the small blue shapes, and the solute is the green.
The solute is more concentrated on the right side to start with, which causes molecules to move across the membrane toward the left until equilibrium is reached.