It looks like the article that you are trying to edit was "checked out". Sometimes this can happen if an administrator opens an article and then leaves the page (closes their browser tab) without "closing" or using "save and close". This leaves the article checked out to that user. As an administrator you can "check-in" the article by clicking the small padlock next to the article title.

You can also check-in all articles on your website at any time by going to "System > Global Check-in" in the backend of your site. This wil remove the padlock from all articles across the site.

Please let us know if you have any other questions or concerns.