A parked domain is a domain that simply redirects users to a different domain.
For instance, if you had both the domains myinterestingwebsite.com.au and myinterestingwebsite.net and wanted myinterestingwebsite.net to point to myinterestingwebsite.com, you would park the myinterestingwebsite.net domain and have it redirect visitors to myinterestingwebsite.com.
Parked domains enable you to have multiple domains displaying the same information, but without having to maintain different websites.