A dolly grip is a person who operates a dolly — a wheeled cart upon which the camera is mounted for moving shots.
The cart may travel along tracks or it may move freely on a smooth surface. The dolly grip is responsible for setting up the tracks and moving the dolly during the shot. A separate camera operator usually rides on the dolly and operates the camera.
Many dollies have a hydraulic arm which adjusts the camera height — this is usually operated by the dolly grip.
The position of dolly grip is considered a highly skilled role. Feature film productions pay well for a good dolly grip.