Bilirubin is a yellow pigment formed from the breakdown of haemoglobin in red blood cells, primarily processed by the liver. It is clinically significant as elevated levels can indicate liver dysfunction, hemolysis, or bile duct obstruction, leading to jaundice. A key association is with conditions like hepatitis or cirrhosis, where impaired bilirubin metabolism results in increased serum bilirubin, necessitating further investigation and management.