Park the domain with a domain parking service. Since you may not have content to put on the site, a parking service puts up a dummy page with ads. When people visit and click on the ads, that's money for you. This method works best with popular keyword domains that get lots of traffic. In most cases, you will not have control over what ads appear on your domains.
The domain name industry is quite similar to the real estate industry in a lot of aspects. There are end users, brokers, consultants and domain flippers or “domainers”. Domain flipping works similarly to buying a house, renovating it (or even sometimes just sitting on it) and then selling it again at a higher price point. The gist of it is: you’re purchasing a domain name and betting it’s worth (or will be worth) more than you paid for it. If you’re right, you get a nice paycheck and move on. Those who make a living out of this just rinse, repeat, and scale.