When a file such as an article is opened for editing it is 'checked-out' and thereby locked for other users to also work on. When checking-in the file this lock is removed, thereby making it possible for another user to open it for editing.
When the Web browser crashes or when a user forgets to check-in the file, the file remains locked. It can be unlocked by using the Abort Checkout option.
How to recognize files that are locked
Files that are locked show a padlock icon in the 'In use by (icon)' column or on top of the thumbnail.
Unlocking locked files
To unlock a locked file, right-click it and choose Abort Checkout.
Note: To unlock files that have been checked-out by another user, you need to have Admin rights.