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.
Dishes, equipment, or surfaces that touch food were not properly washed, rinsed, and sanitized, or sanitizer was not at the right strength. Clean surfaces help stop germs from spreading to food.