When working collaboratively on a project, many users could access a data quality item (being an analysis, a report, a pattern, a rule, a connection, etc.) in the shared repository 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 (it opens in a read-only mode) for all other users.
If you click the Refresh button in the upper right corner of the DQ 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 open these items. Items opened or locked by you have a green lock docked on them.
Your Talend Studio provides several lock modes that permit to grant the "read and write" rights to one of the simultaneous users of the data quality item.