How are you confused? More detail about how you are confused would be helpful, but maybe the following will help get you started.

I wrote a couple short articles about the relationships between shutter speed and aperture. You can find them at https://fotodad.wordpress.com/2012/1...d-photography/ and https://fotodad.wordpress.com/2012/1...n-photography/. The first one gives you a quick overview on the relationship between ISO, shutter speed, and aperture. The second one goes more into apertures and why they follow a seemingly odd sequence.