Pending changes is a way to configure a page so that it displays to readers only edits that have been reviewed.
What it means for a page to be pending changes protectedEdit
On a pending changes protected page, edits by unregistered (or new) users are not shown to readers until reviewed by a pending changes reviewer. This is thus a lesser form of protection than semi-protection.
When is a page pending changes protectedEdit
A page is pending changes protected by an administrator when it matches specific criteria set forth by the wiki's protection policy.
How to review pending editsEdit
All pending edits appear at Special:PendingChanges, which provides the option to list only those from pending changes-protected pages (or actively deferred, if deferred changes is enabled), and can be reviewed by users with review permission.