It’s something akin to picking stocks. You want to buy undervalued domains, and sell them later on at a higher price. For example, you can pick a domain that is out of favor, but could be related to some future event. So if you decide that the stock market is likely to crash in the future, you can buy a domain that includes the words stock market crash during a rising market, and then sell it in a falling market.
One of the cool things about Google AdSense is that it's so easy to get set up. If you have a blog or website, you can sign up for a free Google AdSense Account. From there, Google will give you a unique code that you will paste onto your website. Google takes it from there, tracking your page views, traffic, and earnings on your behalf. There is no upkeep or maintenance to get this thing going, which makes it a no-brainer if you have a website already.