At any given time you may see a padlock next to a specific item in Joomla!'s administrative backend. These padlocks may be displayed next to any of the following (Content Items, Menu Items, Modules, etc).
The Joomla! system places these padlocks next to an item to indicate that a user is currently editing the item (i.e. the item is "checked out"). The lock is removed when the user clicks on the "Save" or "Close" button for that item. If the user never clicks "Save" or "Close" and instead hits the "Back" button or navigates to another page, then the item remains locked. If a different user needs to work with an item that is in the "checked out" state he or she must first ask a manager or administrator to check the item in before any new changes can be made.
There are three ways of checking items back in. One way is to contact the person that has the item checked out to see if they are done with the item. You can hover the cursor above the lock and the tooltip popup will display the name of the user who checked out the article.
If you have access to the administrative backend and sufficient rights and can also either click on the padlock to check only this item back in or click on Site->Maintenance->Global Check-in to check all items in.
The later option should be used very carefully, especially in multi-user environments. This single action checks in all previously checked out items, whether they were checked out by you or not. Possible undesirable side effects may be that multiple editors end up working on the same document. In this case whoever clicks the save button last has their version saved as the final copy.