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