Automatic Locking and Unlocking
When somebody clicks on the Edit tab, that item immediately becomes locked. This feature prevents two people from editing the same document at the same time, or accidentally saving edits over another users edits. In this example, George Schrubb has started editing the "Widget Installation" document. When Jane Smythe (who has permissions to edit that document as well) goes to view it, she will see the following:
Once George has finished editing the document and clicks the Save button, the document is automatically unlocked and available to be edited by others (should they have the proper permissions to do so, of course).
However, if it becomes clear to Jane that George isn't really editing the document anymore (e.g. the locking message says that the item was locked several days ago and not just a few minutes ago) then Jane can "unlock" it and make it available for editing again.
In Plone 3.3 or higher:
Should you desire to disable locking, go to the Plone control panel (Site Setup -> Site) and uncheck Enable locking for through-the-web edits.