Parking actually refers to two different things. The first, is simply putting up a page telling users that it's under construction, or that a site is soon to come, or redirecting to an existing site, etc. But when people most often refer to parking they are talking about cash parking. Basically, with cash parking, you use a service that places ads on the pages, and you hope people click on the ads. If someone does, then you can get a few cents with every click. If you have a domain that gets thousands of visitors this can end up being lucrative. If you don't get that many people visiting the site however, you will not see much revenue from it, if any.
http://en.wikipedia.org/wiki/Domain_parking http://www.fka200.com/domain-parking -> PARKING PROVIDERS! Not information on what it is... although I think it would be smart for me to write something about domain parking.