You've probably come here from a link on a protected page, which cannot currently be edited, or a protected image which can not currently be overwritten. Administrators have the ability to "protect" pages or images such that they cannot be modified except by other admins. This ability is only used in certain circumstances.
Reasons for protecting pages include (but are not limited to):
- Protecting high visibility pages such as the Main Page from vandalism.
- Maintaining the integrity of the site's logos.
- Protecting the often-used texts in the MediaWiki namespace.