Primary active transport uses a protein pump embedded in the cell membrane. When a molecule of ATP binds to the pump, it breaks down (into ADP + phosphate), releasing energy. That energy changes the shape of the pump, forcing a molecule to be shoved across the membrane—regardless of which direction it wants to go.

In secondary transport, a molecule (like sodium) naturally wants to flow back into the cell (down its gradient). A co-transporter protein lets that sodium ion fall back in, but only if it brings a "passenger" molecule (like glucose) along for the ride—even if the glucose is moving against its own gradient.

Think of a hydroelectric dam. The water stored up high (created by primary transport) has potential energy. When you let the water flow down, you can use that flow to power something else (like a turbine).