Hello! :) I'm going to explain active transport (and also passive transport) in simple terms.
The concentration gradient is the movement of molecules from an area of high concentration to an area of low concentration of the molecule.
Passive transport happens when the molecules go down the concentration gradient, but what would happen if a cell needed to move molecules against the concentration gradient?
The process of moving molecule(s) against the concentration gradient is active transport. Active transport requires energy, however.
There are multiple types of active transport, such as endocytosis and exocytosis. Endocytosis moves molecules against the gradient into cells, while exocytosis move molecules outside the cell. There are two types of endocytosis: pinocytosis (liquids) and phagocytosis (solids.)
Hope this helps!