A parked domain is a domain that has been purchased, but is not yet pointed to a website.
Rather than show an error, Enlighten Domains simply shows a more friendly "Parked" page. This is a free service, and all newly registered domains are "parked" until they are configured for use.
Enlighten Domains also provides free Domain URL forwarding, so clients may consider using this feature to redirect users to another site (possibly a Facebook page or similar) while the final site is being developed. More information about this can be found here: URL Redirection