A protected page is a page where normal users are prevented from editing at all. Only a user with sysop permissions can edit 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. Typically a protected page is one which has been repeatedly targeted with vandalism, or where it is believed that vandalism would have a unusually severe impact.
For sysop information see Help:Protecting and unprotecting pages