When a user is editing a content item, module, menu item etc., Joomla "locks" this item to other users. This insures that changes can not be made by different users at the same time. Below is a picture of what another user would see in the backend if he were to try to also edit the setting of the "Who's Online" module.
If the user who is editing the module (for example), did not hit the "Save" or "Close" button, and simply closed the browser, then the item will be locked until that user returns to that item and clicks one of these buttons. This can cause problems if you want to edit an item and not have to have the other user involved in doing so.
Joomla has an easy way to unlock all the items. On the top menu, select "Global Check-in" in the "Tools" menu as seen below.
This checks all item unlocking them to allow any user to edit them again. The following picture shows what you will see when an item is checked in
It is that simple. Remember, if you are having problems being able to edit a content item, try this option. It may be due to the content item being locked.