If you're just getting started on your filmmaking journey, one of the most crucial cinematic concepts to understand is exposure. Sure, your camera most likely has an auto setting that helps you expose your image correctly, but that's not going to be helpful in every shooting situation, which is why you should understand the different tools filmmakers use to achieve a proper exposure: ISO, shutter speed, and aperture. Caleb Pike of DSLR Video Shooter provides an excellent explanation of these all-important camera settings in the video below:
So, there you have it. It might be complicated at first, especially when you take into account how some settings offset the others, but once you get a handle on what each one does and how they change your image individually and collectively, it's actually pretty straightforward and simple.
One important note: be sure to have plenty of light to create your images. Adjusting your ISO, shutter speed, and aperture to make up for lack of light is sometimes a necessity, but it shouldn't be your everyday solution, especially when it doesn't have to be. Shooting a doc in the middle of the jungle at night—makes sense. Shooting a film in your backyard at night—plug in a light!
Source: DSLR Video Shooter