Answer:
Mitochondria are like the power plants of the cell, similar to the engine of a car.
Mitochondria are organelles and they convert chemical energy (from food) into ATP, the main energy source for cellular work. Mitochondria suck and take in nutrients, break them down, and use it to make energy through the process of cellular respiration. One single cell has many mitochondria (1000 - 2000). This number really depends on the function of the cell.
For example, a nerve cell needs less energy than a muscle cell, and therefore will have fewer mitochondria. Kind of like a fast car needs a bigger engine than a slower car.
Also,
The mitochondria are closed in by two membranes. The outer membrane is the one that covers the organelle, and the inner membrane forms a special compartment with a fluid called the mitochondrial matrix. The inner membrane contains many folds called cristae, which increase the surface area inside the organelle. This increases the efficiency of the chemical reactions, allowing the mitochondria to create more ATP.
Hope this helped!