Cellular respiration is a chemical process that releases energy from glucose. It is a universal process, which means it happens in all living cells, including animal, plant and fungal cells. It goes on continuously, day and night, for as long as the cell is alive.
The energy is used to make a small molecule called ATP. ATP carries energy to the parts of the cell that need it. Cells use ATP for movement, for building large molecules, for active transport and for keeping the chemical reactions of life going.
Do not confuse respiration with breathing. Breathing (ventilation) moves gases in and out of the body. Respiration is the reaction that happens inside the cells. A plant cell in a leaf respires at night, when it cannot photosynthesise, so it still has energy to use.