The camera needs a certain amount of light to create a perfectly exposed image--not too bright, not too dark. Your job is to ensure the camera gets the light it needs. Let's use a bucket analogy. Your camera needs a bucket of light, and you have three mugs to pour light with--aperture, shutter, and ISO. Their contributions don't have to be equal. What matters is that the bucket is full. If it’s not full, your image is too dark. If the bucket overflows, it's too bright.This is popularly known as the exposure triangle, but I find the bucket analogy easier to understand.