While most articles can be edited by anyone, semi-protection is often used on more popular pages, such as featured pages.
The reason for protection can be found in the protection log. If there are no relevant entries in the protection log, the page may have been moved after being protected.
What can I do?
If you have a user account, log in first. If you do not yet have an account, you may create one; after 4 days and 10 edits, you will be able to edit semi-protected pages.