The Joomla system places padlocks next to an item to indicate that a user is currently editing the item (i.e. the item is "checked out"). If the user never clicks "Save" or "Close" and instead hits the "Back" button or navigates to another page, then the item remains locked.
This task-plugin automatically checks in all articles that are checked out longer than you defined. and with the PRO version you can also check in categories, modules, tasks, plugins, etc.