Question

What is active transport ?differentiate between active and passive transport ?

Answer 1

I am going to assume you mean transport through a cell membrane.
Active transport is when a cell actively pulls different things from the low-concentration outside the cell to the high-concentration inside the cells. Think of it as blowing up a balloon, it takes work to introduce more air inside because there is more pressure inside the balloon that wants to escape

Passive transport is when a cell allows things to move in and out of the cell membrane without taking any energy. The concentration of a substance on the inside of the cell and on the outside of the cell is equal.

Answer 2

is that why they say active transport is the movement of molecules across the cell membrane against a concentration gradient?