The electron chain is a series of complexes that transfer electrons from electron donors to electron acceptors. Think of it as a game of hot potato...electrons are passed like this along the chain. This electron transport is coupled to the movement of protons (H+) across the membrane. As more and more H+ is moved across the membrane, this creates a proton gradient (higher H+ on the outside of the membrane) that drives the synthesis of ATP (adenosine triphosphate). So...this was the long winded answer to your question: ATP. It is a very important concept in biology.