Hot TCS food item that has been cooked and cooled is being held for service without first being reheated to 165º F or above within 2 hours.
In plain English
Food was not kept cold enough, hot enough, cooled fast enough, or cooked or reheated to the right temperature. Bacteria grow quickly in the "danger zone" between 41°F and 135°F, which makes this one of the most important food-safety checks.