The electron transport chain is a series of proteins embedded in the inner membrane of the mitochondria.
As electrons move through the chain, it’s like rolling down stairs, as they roll, electrons release energy.
As they reach the end of the chain, they join oxygen to form water.
The energy released is used to pump H+ ions across the membrane into the intermembrane space.
These H+ ions are being pumped against their concentration gradient,
so they are being moved from an area of low concentration to an area of high concentration using the energy from the electrons
Those H+ ions build up in the intermembrane space and eventually rush back through the inner membrane to try and equilibrate.
They move back through a protein called ATP synthase,
and the movement of the H+, spins the ATP synthase. That spinning motion is used to create ATP.
This is where the great majority of the ATP produced bycellular respiration is made.