|MediaWiki Handbook: Contents, Readers, Editors, Moderators, System admins, Researchers +/-|
When a page is protected, certain users can't edit it. Semiprotection disallows IPs and new users from editing a page, while full protection stops anyone but sysops. However, no matter what level of protection a page is given, any user is free to view the source, an action that is denoted view source where the edit tab typically is. In this way, anybody can copy and examine the wikitext of a page to their heart's content, but no changes can be made.
The appearance of view source is not limited to inability to edit due to page protection. For example, users without the editinterface permission will see "view source" when trying to edit a page in the MediaWiki namespace.
The view source form is shown below.