When working collaboratively on a project, many users could access an item in the shared repository or at the project level simultaneously. In such a case, the user who opens the item first will have the "read and write" rights. The item will then be locked and read-only for all other users.
If you click the Refresh button in the upper right corner of the Repository tree view, the items that have been locked by other users will have a red lock docked on them. You will not be able to make changes to these items.
Items stored in the Repository tree view that are submitted to lock/unlock system include:
Metadata of various types (DB connection, File...),
Other items such as documentations, CDC, etc.
Items at project level are also submitted to lock/unlock system. These items include all Project Settings.
Talend Studio provides several lock modes that allow to grant the "read and write" rights to one of the simultaneous users of the repository item.