Fouling is caused by the buildup of material inside the barrel and chamber. Various materials contribute to fouling, including lead, copper, plastic from wadding, and carbon from the combustion process. As fouling occurs, it causes your rifle to become less accurate.
Another concern with fouling is that it can cause serious damage to your firearm. Left unaddressed, it can lead to rust, pitting, and eventually to parts getting fused together. Removing fouling is the main reason you need to clean your AR-15.