When a doctor is listening to the back of your body with a stethoscope they're comparing the sound of your lungs from the front to the back as the lungs can sound different when listened from the front part of your body and from the back part of your body.
The medical term for this is Auscultation.
The airflow through your lungs sound differently when airways are blocked, narrowed, or filled with fluid so they listen from front to the back to get different results.