What is Exposure?
Imagine yourself getting a sun-tan. In the morning when the sun power is weak, it will probably take a couple hours before you get a sunburn. At noon, it will probably take less than half an hour for you to get the same burn (ok, tan) because the sun is much more powerful. Now imagine sun power as aperture, and the time it takes to burn yourself as shutter speed. Get it? Then… a third variable is skin sensitivity. People who are usually under the sun (like a surfer) are probably less sensitive to sun power; therefore, it will take more time to get a burn. So skin sensitivity is similar to ISO. A surfer would be ISO 100; while Ginny is probably ISO 800 (more sensitive).
No a little more technical. How does the camera know how much exposure is needed? Some how, some genious researches concluded that 18% gray (imagine mixing 82% white paint and 18% black paint) is the optimal exposure for all pictures. So the camera sensor will try to calculate the correct aperture, shutter speed, and ISO based on average 18% gray brightness. This is also called “metering”. It does a very good job most of the time. It’s when you go outside of 18% that you start to get into interesting pictures. Try it.