A burn unit is a place in the hospital that has nurses and surgeons specializing in the treatment of burns and more severe burns.
If you get burned badly and get sent to a hospital they will do what they can to treat you but if that hospital does not have a special burn unit then they will usually life flight you to another nearby hospital where they do have a burn unit.
Some burns can be treated at most any hospital but other more severe burns that may threaten your life may need more specialized treatment.
The severity of a burn, and therefore whether a referral will be made after the patient is treated and stabilized, differs depending upon many factors, among them.
Also the age of the victim (burns to infants and toddlers or to those over age 65 are generally more serious, particularly if the face, head, respiratory system, chest, abdomen, groin, or extremities are burned; those who are not in these age groups can be more affected if they are or were already ill, injured, or immunocompromised)
The doctors in the burn unit have more training and knowledge about treating more severe burns so they have a special burn unit.