ETC is a series of proteins in the inner mitochondrial membrane. Electrons get transferred from one protein of the series to another and these electron transfers (or redox reactions) pump H+ out of the mitochondrial matrix into the intermembrane space. Thus there's a lot of H+ outside compared to inside the mitochondria. Because of this H+ gradient, the last protein of the ETC, which is an ATP generator, will allow H+ to "slide down" the gradient back into the mitochondria and through this process generate energy or ATP by adding a P group to ADP.

For the MCAT, just know that ETC generates a H+ gradient through electron transfers or redox reactions, and it is this gradient that allows the generation of energy or ATP. ETC is essential for oxidative phosphorylation - using intermediates obtained from glycolysis and Kreb cycle (NADH and FADH2) to give us energy. This process gives us a lot of energy from glucose compared to in anaerobic respiration where only glycolysis occurs. Also remember that the terminal electron acceptor (TEA) of the ETC, where the transferred electrons end up, is oxygen O2 (that's why we breath and need O2!), which upon accepting the electrons turns into water H2O. So if any step along this process is malfunctioning, for example lack of O2, then ETC and oxidative phosphorylation stop working.

Yang