Josh Cripps has many really helpful videos on his YouTube channel. In this one he will help you get a handle on one of the elements of the exposure triangle that is often confusing and perplexing to many beginners – ISO.
Watch as his gives real world examples, learn what ISO is, when to change it, when you should raise it or lower it and why. Ultimately having a better grasp of ISO will help you take better photos, and get the results you want.
He also mentions his teacup video where he explains the relationship of aperture and shutter speed. You can see that one below for your reference as well:
I hope these videos have given you a good idea about ISO and how to use it in your photography.