According to gorillafilmonline.com, the grip is the person in charge of setting up equipment to support the camera, and on some sets, support lighting equipment (but not the actual lights. Never touch the actual lights). It’s a physically demanding job where experience is invaluable.
The grips set up all the rigging which allow the camera to move about within a set in a way that captures the shot as the director wants it, so this can involve working on camera dollies, cranes, tracks and other camera setups. When a grip does his job well, the camera movement through the scenes will be seamless, so much so that you don’t even think about it, meaning you don’t think about the grips.
The grips below are moving a camera dolly onto a track. I hope they have a chiropractor on set. Or at least a masseuse and a couple ice packs.
Reelrundown.com states that the term “grip ” dates from the days of silent movies when cameras were hand-cranked. The cranking action would make the camera wobble, and so the camera operator would call for anyone with a “good grip” to grab the camera tripod legs and physically steady the camera.