Page permissions, or more accurately restrictions, prevent access to a page (and all its children) to all but the named users (and groups). It's important to note that page restrictions cannot grant access to users. Restrictions are set from the Edit screen of a page, and there are only two: view and edit.
Restrictions limit page privileges to the only named users and groups
Privileges can only be added at the global or space level.
Because of this, page restrictions are useful in more limited circumstances. One example is allowing a team to collaborate on draft pages in a space without being read until the new section is ready to be published.
An important consideration is that page restrictions can be added or removed by any editor, where space permissions are set only by administrative users.
Example
Lorem ipsum dolor sit amet, consectetuer adipiscing elit.
Aliquam fermentum vestibulum est. Sed quis tortor.