Plain English Meaning
It is a signal from the airport that tells the pilot whether they are left or right of the runway centerline while flying away from the runway, used after a missed approach or on departure.
Full Definition
In a Microwave Landing System (MLS), back azimuth is a guidance signal that provides lateral course information to aircraft after a missed approach or during departure, projecting outward from the departure end of the runway. It allows the pilot to fly a precise centerline track away from the runway, opposite in direction to the front azimuth signal used for the approach.