What does a mitocondrion do

The mitochondria take fuel (glucose) and essentially burns it. The energy given off by oxidizing the fuel is harnessed by causing ions to move across a membrane, kind of like pumping water. The ions then flow down through a protein which synthesizes ATP much like the water then flows down through a dam to generate electricity.

A mitochondria is basically the power house of the cell. It turns left over glucose into energy.