A widely-researched phenomenon in the Middle Ages, phlogiston was thought to be an inherent substance in all things that could burn. When something was burned, phlogiston was visible in the form of flame, and was released into the air. In turn, people thought something would not burn without air because the air was already "full of phlogiston" and no more could be released into the air. In short, phlogiston was believed to be the substance that fire was made of.
These days, under influence of modern science, the concept of phlogiston is usually regarded as superstition.