An electron transport chain(ETC) couples a chemical reaction between an electron donor (such as NADH) and an electron acceptor (such as O2) to the transfer of H+ ions across a membrane, through a set of mediating biochemical reactions. These H+ ions are used to produce adenosine triphosphate (ATP), the main energy intermediate in living organisms, as they move back across the membrane.
In mitochondria, it is the conversion of oxygen to water, NADH to NAD+ and succinate to fumarate that drives the transfer of H+ ions.
Source: [Wikipedia](https://en.wikipedia.org/wiki/Electron_transport_chain)