When you work with a small team to make a short film, like you and your friends, you don't have a Key Grip and you might not know what it is. When you work with a big team, you have so many new roles that you might have no idea about. Key Grip is one of them and it is also very important. He is the boss of the grip department and he helps with the gear. I mean they literally move the tripods, dolly...
A movie is made with an entire crew. The public mostly knows the actors because they are more visible on-screen. However, behind the scenes, there is an entire team and Keygrip has a very important role. The grip team is the one who sets up the whole equipment on the movie set. The Keygrip is their leader.