A parked domain is a domain that doesn't have a hosting account associated to it, and that is usually enabled with URL forwarding capabilities, so that it points to an existing website.
For example, let's assume that you already run a newsletter that is hosted in a subdirectory of your domain name, as follows: "http://www.domain.com/newsletter/index.html". You may at one given point want to register a separate domain name for your newsletter, so that it is more memorable, but may not want to move its pages to a new server, open a new hosting account, or pay to establish an add-on domain. You can then register a and park a new domain for your newsletter (for example: "http://www.newsletter.com"), which will be forwarded to "http://www.domain.com/newsletter/index.html".