A protected page is a page where normal users are prevented from editing and/or moving at all. Only a user with sysop permissions can edit or move a protected page. Likewise only sysop users can protect a page in the first place, or unprotect the page to lift the restriction.
There are several reasons why a particular page might be protected. See Communpedia:Page protection.
For sysop information see Help:Protecting and unprotecting pages