Since Plone 3.1 content edit locking has been supported. This feature is to prevent simultaneous conflicting edits of the same content.
If the editor forgot to press Save or Cancel explicit unlocking must be performed on locked objects if you want to modify them. Unfortunately the side-effect is that if site has objects left to locked state they usually interfere with your programming.
Since Plone 3.3 the feature can be disabled from the site settings.
Unlocking can be performed from the object view or edit tab.