Domain parking refers to a method people use to reserve a domain.  Typically what happens is someone registers a domain name because they want to reserve it, but they either don’t have the website ready or they just want to be able to use email addresses at that domain without actually having a website.

Domain parking consists of registering a domain and having that homepage consist of a page that looks like an error or a page full of advertising or other relatively non-helpful information.  Sometimes a simple “under construction” page is used, even if there is no intention of making the page better in the future.