The leading edge of cold air which has descended from a Cumulonimbus (Cb) cloud and is moving horizontally near to the surface into warmer air.
As a Cumulonimbus cloud becomes mature, strong downdrafts occur, usually associated with precipitation. When the cold airflow hits the ground it moves outwards away from the cloud. Where this mass of cold air meets the surrounding warmer air, conditions similar to a Cold Front can occur. The gust front may push up the warm air ahead of it creating a wall of cloud as the moisture in that warmer air condenses as the colder air penetrates. Gustnados, relatively small, but nonetheless destructive vortices may develop along the gust front.