1. What does this padlock mean:
Joomla Core Components like content (articles), categories, menu, modules, extensions, etc, contain in their database table structure a field called "Checked-Out", which can take 2 values: "0" for Checked-In status, or the actual user ID for Checked-Out status.
When any user is editing a specific item e.g. an article, Joomla's system changes this item's "Checked-Out" value from 0 to the this user's ID, which means that the item is currently in "Checked-Out" status.
The item will revert back to its "Checked-In" status (Checked_Out = 0), when the user clicks on the "Save" or "Close" buttons of the item.
During the Checked-In status, Joomla displays this Padlock icon next to the item's title in the items list, to indicate that the item is currently being edited by a user.
Furthermore, Joomla won't allow any other user to access the same item simultaneously.
This is to prevent undesired results and confusion in the backend, as if more than 1 user would be able to edit the same item at the same item, only the changes of the last user that would click the "Save" button would take place.
An interesting side note here:
A padlock may appear even when no other admin user is logged in, or even when the site has only 1 administrator.
As said an item will enter in Checked-Out status, when being in Editing mode. The status will only revert back to Checked-In, when the user click on the Save or Close button.
These buttons are triggering the functions to update the database. If for any reason the user leave the editing mode of this item, without clicking those buttons, then the item will continue to have the value of the User ID for the Checked_Out field in the database, which will make Joomla believe that this item is still under editing mode.
2. How to unlock the item.
Usually in multi-user environments, you will want to wait for the other user to complete his work on this item. It's recommended to contact him and ask him about the status of his work.
You can find who is the user that edits the item, by hovering with your mouse over the padlock, and a tooltip with the relevant info will appear.
To unlock the item, you need to have backend administration access and the required permissions to access the Checked-In functionality.
1 Item Check-In
In the back-end administration items list (e.g. articles list), you can either click on the padlock icon to unlock that specific item.
Multiple Items Check-In of the same List
If there are more than 1 locked items that you want to unlock in the list, you can tick their checkbox and then click the Check-In button, on the toolbar.
Global Database Check-In
Finally there is a Global Check-In, under "System" Menu, where you can Checked-In all database tables that supports the Checked-Out feature at once.
Be aware though, that this should be used with caution, especially in multi-users sites, to avoid any issues/confusions as described above.