DocSecrets places color over certain parts of the doc that are password protected by the owner. After the owner installs and clicks on DocSecrets from the add-on bar, he/she is asked to create a password and then gets to choose what needs to be private from other users. When other users open up the document, the privileges granted by the owner will stay the same, but the text that was marked as private by the owner will not be viewable. The owner can then decide to make this text viewable as needed.
Click here to learn more about this new add-on.
Google Guru's overview and video