This is a topic for much more involved discussion than ELI5, but in the simplest of ways, the body breaks down the food to the smallest molecules possible – those then get absorbed into blood and transported to cells around your body. The cells then take up the simple molecules and specialized organelles inside the cells conduct chemical reactions, with help from enzymes, that oxidize those molecules (literally “burn” them, but in a slow, molecular level) which produces CO2 and water, and releases energy, that energy (in the form of an electron) is captured by other molecules in another reaction and used by cells that need it, or stored as fat.
Latest Answers