If a substance moves down its concentration gradient, it means that it is moving from an area where it has a high concentration to an area where it has a low concentration. This is known as diffusion.
A concentration gradient refers to the difference in solute concentration between two regions. When a solute moves from an area of higher concentration to lower concentration, it is moving "downhill" along the concentration gradient. In the reverse situation, where a solute moves from an area of lower concentration to higher concentration, it is moving "uphill" against the concentration gradient.
Simple diffusion
its active transport because it expands in opp direction and pushes
Passive transport involving facilitated diffusion moves particles up a concentration gradient and requires a carrier protein to facilitate the movement across the cell membrane. This process does not require energy input from the cell.
The main difference is that diffusion does not require energy and active transport does require energy. Just for the record diffusion has a concentration that moves from high to low, and active transport has a concentration that moves from low to high. (Hope this helps to answer your question!)