A parked domain is a domain that points to your main/default domain document root directory.
For example, suppose you have 2 domain names, domain1.com and domain2.com. When you registered with Farbyte you setup a main domain. In this example we'll assume that domain1.com was the registered domain.
If you park domain2.com it will point to the same document root directory as domain1.com, thus users navigating to domain2.com will see the same content as domain1.com.