This movement is called diffusion, and it's a passive process meaning it doesn't require energy. The concentration gradient is simply the difference in concentration between two areas, and molecules naturally tend to move from the area where they are more concentrated to the area where they are less concentrated.
Think of it like this: imagine you spray perfume in one corner of a room. The perfume molecules are highly concentrated in that corner. Over time, the perfume molecules will spread out, moving from the high concentration area to the rest of the room where the concentration is lower. This is diffusion driven by the concentration gradient.
