The Key Grip plays an essential role on set.
The Key Grip, the Gaffer and the Cinematographer work together. They supervise the grip crews and report the progress to the DoP. The Key Grip needs problem-solving skills, technical knowledge about the gear and strong communication skills because he needs to coordinate the crew. The Key Grip also coordinates the transportation of the equipment and is in charge of its set-up.
Simply put, the Key Grip is the person who tells the Grip Department what to do, where to move the equipment to, install the tripods, install the dolly, move the dolly, maintain the rigging and equipment that supports the camera, process trailers, lights and by lights I mean stands, diffusers etc.
The key grip is a member of the camera crew responsible for setting up and maintaining camera-support equipment such as dollies, cranes, tracks, jibs and other specialized rigging. They are usually the first to arrive on set and are considered one of the most important crew members.