Mitochondria is a cell organelle which is also called the powerhouse of the cell.
Some processes of cellular respiration take place here, like the Kreb cycle and oxidative phosphorylation.
By the reduction of many compounds, energy is produced in the form of NADH2 and FADH2.
These molecules are finally converted into ATP.
ATP is a molecule that has high energy bonds. So these are also called the energy currency of the cell.
Whenever energy is required in the cell the phosphate group from ATP is transferred to other compounds and high energy is released due to the breaking of the phosphodiester bond.