I don't know what to call it... I was thinking about "The Triangle of Relativity" or "The Relative Triangle" or many others that would be better off not written down. Anyway, the key to making money with Google Adsense is this:

1) Have a website with lots of content about one subject.

2) Put Google Adsense on your site such that the ads relate to your content.

3) Get people who are interested in your content to come to your site.

Each of these three things is worthy of hours of discussion, but it is important to not lose sight of the fact that if any one of these things is missing, your chance of making a lot of money with adsense is slim. Let me explain:

If you have a site about subject X and your visitors are interested in subject X, but your ads are about subject Y, then your visitors will probably not click on your ads.

If you have a site about subject X with ads about subject X and your visitors come to your site thinking they are going to find subject Y, but find subject X, they will leave.

If you have a site about subject X, and your ads are about subject X and your visitors come because they are interested in subject X, then there's a very good chance that the ads will be interesting to them and they will click.

There are a lot of other things that go into being successful running an adsense website, but this is the key. If you don't understand this, nothing else matters. That's not to say that nothing else is important, just that you HAVE to keep this concept top-of-mind.