At all levels of care, the ABC protocol exists to remind the person delivering treatment of the importance of airway, breathing, and circulation to the maintenance of a patient's life. These three issues are paramount in any treatment, in that the loss (or loss of control of) any one of these items will rapidly lead to the patient's death.
The basic application of the ABC principle is in first aid, and is used in cases of unconscious patients to start treatment and assess the need for, and then potentially deliver, cardiopulmonary resuscitation.
In this simple usage, the rescuer is required to open the airway (using a technique such as "head tilt - chin lift"), then check for normal breathing. These two steps should provide the initial assessment of whether the patient will require CPR or not.
In the event that the patient is not breathing normally, the current international guidelines indicate that chest compressions should be started.