While the error is fairly self-explanatory, it's not always clear how to overcome it and in particular WHO can overcome it. The basic rule is that if a user has opened an article or module for editing it can't be opened by anyone else until the original user closes it.
However, problems arise when the original user does not close the article or module properly. If the browser page times out or is simply closed before the article or module is closed within the Joomla Administrator, the lock will remain in place until it is cleared somehow. This is normally done by having the user who was editing the item or a Super User open it and close it properly, or with a Global Check-in.
If a Super User has locked an item, only that user or another Super User can clear that lock. Also, Super Users are the only users who can perform a Global Check-in.