Within the context of this discussion, domain parking refers to the process of adding additional domain names to a hosting account with all of the domain names resolving to the account's index page.
For example:
abc.com -> http://abc.com/index.html
def.com -> http://abc.com/index.html
It is very common for companies that purchase rights to mydomain.com, mydomain.net, mydomain.org, etc., to have users land on the same index page when they visit the website through any of the domains.