The pump refers to how your muscles increase in size during a workout, and feel tight.

This occurs because you are forcing blood into the particular area you are activating.

The blood supplies the muscles with oxygen, which is needed for the muscles to perform at their peak.

By giving the muscles enough oxygen to work effectively and recuperate between sets, it means you have more energy to exert into your workout.

This is useful because it enables the muscle to grow. Also, during the pump, the fascial layer between the muscle and skin expands to make room for more potential muscle.