Sometimes, if more people work on the site, you can get locked out of a certain module or article because the site thinks someone else is still editing that item. When opened, each Joomla item is checked out, this way Joomla protects each editable item from being edited by two separate users at the same itme, and this way avoiding potential confusion and other obvious problems.
When the editing user quits normally the process, the item gets checked in back. But sometimes, when the process isn't ending by the user normally (but clicking one of save, save&close, save&new, close or similar buttons, depending on Joomla version and edited item) the items are remaining checked out. This happens also when user session gets closed without the above actions taking place (timeout, browser or connection forcefully closed, browser/OS frozen. etc.).
That is very common and it is not a big deal. If you have enough user level - and if you are playing with the administrative backend, you probably have - you just have to go in the administrative side of your site and hover over the "Tools" menu item and select "Global Check In." Now Joomla! knows exactly what is being edited and if that article or section was locking you out before, but not really being modified, Joomla! will unlock it so you can get back to business.