What is active transport?

(N/A) $\Rightarrow$ Active transport is the movement of ions or molecules across a cell membrane against their concentration gradient,i.e.,from a region of lower concentration to a region of higher concentration.
This process requires the expenditure of metabolic energy in the form of $ATP$.
It is essential for maintaining specific ion concentrations within the cell,such as the $Na^{+}/K^{+}$ pump,which actively transports sodium ions out of the cell and potassium ions into the cell.