When you register a domain name, you can either "park" the domain or you can upload your own web page(s) to the domain. In other words, if you don't upload your own pages, the domain remains parked. Parked pages will usually automatically display ads that generate profit for either you, for the parking company, or both.
If you simply register the name and do nothing more, often it is automatically parked and the registrar get all the click revenue. But if you sign up for a parking program, you can use the parking company's nameserver settings, and they will pay you a percentage of all the revenue generated when people click on the ads that are displayed on your parked domain.
And know that you do not have to use your domain registrars parking service. You can register your domain and use the parking service of any one of many available parking companies. For example, you can register your domain at Godaddy, and choose to have a parking page from NameDrive, or Fabulous, or Sedo, or many others. You can switch the parking service any time you wish, to see which parked page generates the best revenue.