The compressor relay's purpose is to start the compressor. A faulty relay can result in the compressor failing to cycle on. The relay is accessed from the lower rear of the refrigerator. In the photo below the compressor is labeled "A", the relay is located in the terminal box which is located at the position labeled "B".
Before testing a compressor relay, unplug the refrigerator to avoid an electrical shock hazard. Also, if the compressor uses a capacitor, it must be discharged first to avoid the risk of electrical shock.
The compressor relay can be accessed by removing the terminal cover box. The cover is held on by tension or with a retaining clip.