Larger boils and carbuncles are best treated by letting the infected fluid (pus) out. Sometimes this is done by a doctor who drains the pus, using a needle and syringe. Sometimes a small cut in the skin is needed to let out the pus (incision and drainage). The wound is covered with a dressing until the skin heals.
https://patient.info/skin-conditions/boils-carbuncles-and-furunculosis