You can demonstrate diffusion easily in your home. Fill a glass with water. Then add 10 drops of ink (any color) to the water very carefully. The ink sinks to the bottom of the glass because it is more dense than water. Place the glass in a place where it will not be disturbed and make observations of it every day. Over time, the colored ink at the bottom of the glass spreads upward. It moves from a region of high concentration to one of low concentration.
Diffusion eventually stops because no region of high ink concentration remains. The concentration of ink and water is the same throughout the glass. That rule applies to all cases of diffusion. When differences in concentration no longer exist, diffusion stops.