The Grip is in charge of whatever the camera sits on; for instance: the tripod, dolly, gimbal, steadicam. Think of them as the people who hold tightly onto things (who grip … geddit??)
There can be specialist Grips: such as a Dolly Grip.
In some countries (e.g. USA), Grips also look after lighting stands. In the UK, Grips only control what the camera sits on.
If there is more than one grip, the Head of Department (HOD) is the Key Grip. The assistant to the Key Grip is the Best Boy.
The Grip works with the Director of Photography.