Here's why:

* Movement against the concentration gradient: The substance moves from a region where it's less concentrated to a region where it's more concentrated. This is the opposite of what happens naturally due to diffusion.

* Requires energy: Because the substance is moving against the concentration gradient, energy is needed to drive this movement. This energy is often provided by ATP (adenosine triphosphate), the cell's energy currency.

Examples of active transport:

* The sodium-potassium pump in nerve cells actively pumps sodium ions out of the cell and potassium ions into the cell, maintaining the cell's electrical potential.

* The uptake of glucose by intestinal cells is another example of active transport.

Let me know if you'd like to know more about active transport!