Diffusion is the movement of molecules from a region in which they are highly concentrated to a region in which they are less concentrated. It depends on the motion of the molecules and continues until the system in which the molecules are found reaches a state of equilibrium, which means that the molecules are randomly distributed throughout the system. An important concept in understanding diffusion is the concept of equilibrium. There are two types of equilibrium. Static equilibrium occurs when there is no action taking place.
Dynamic equilibrium occurs when two opposing actions occur at the same rate.
For example, consider a bucket full of water. It is in a state of static equilibrium because the water level stays the same. The water is not moving. If you were to poke a hole in the bottom of the bucket, water would leak out. However, if you were to begin pouring water into the bucket at the same rate that it was leaking out, the water level in the bucket would stay the same because the rate at which the water is entering the bucket is equal to the rate at which it is leaking out.