A follow focus is a mechanical device that uses gears to make the rotation of a lens' focus ring easier and more precise for the camera operator or focus puller. It consists of a knob (or two) attached to a body that mounts on rods beneath the lens. A cog on the follow focus meshes with the teeth on the focus ring of the lens. Rotation of the knobs is transferred to the lens ring through a gear mechanism. More about follow focus devices on: https://lensnotes.com/
The follow focus is the key tool of the focus puller. A focus puller, also known as first assistant camera or 1st AC, is a key member of the camera department in film and television production. As the name implies, the main responsibility of the focus puller is to keep the camera lens in proper focus throughout filming. While this might sound mundane in the era of autofocus, it is anything but. This is a highly skilled job, that requires concentration, precision, a keen eye for distances and a superb feel for tempo.