This is the most common setup for lighting on any given film set. This is used to create a global soft light and depth on a subject. I'll walk you through the 3 different lights and their purposes.
1. Key Light: This is the largest & most powerful light. This is normally placed near the camera at a different angle. This is the only light where you should control it's visibility with the barn doors. Change it to however you'd like. It normally has a diffuser so the light illuminates softly. The purpose is to illuminate the scene and give the subject an eye light reflection (they'll look dead without it). It should be placed at eye level or a little bit above. I recommend this Key
2. Fill Light: Since the Key is at an angle, there's going to be harsh shadows on your subject. More specifically, face details like a nose will give one-half of the face a large shadow. The fill light is placed at an angle that will fill in those shadows. The light can be harsh or soft, but it needs to be at a low intensity so it doesn't compete with the Key. The height of the Fill light should be on par with the Key, but it's fine if it's not. I recommend this Fill Light
3. Back Light: The back Light is used to give more depth to the frame. It is placed behind the subject and gives them a small outline that distinguishes them from the background. The intensity shouldn't be too high otherwise, a silhouette would form. The light should be harsh, so don't add any diffusers. The height and placement can be anywhere on set as long as it's behind the subject. Often, the Back Light is placed above the height of the subjects to make sure the light is not in the frame. I recommend this Back Light
Don't forget to leave the lights in the same color temperature. Happy Shooting!