I don't know if you can hide workbooks per se, but I know that you can hide worksheets. You could potentially hide all the worksheets in a workbook and only display the dashboard that uses the worksheets, maybe that is what you are thinking of?
They are only sort of hidden. If you unhide them, even temporarily, anyone who follows you will see the viz in their "Updates" section. Furthermore, hidden vizzes are still accessible if someone has the URL. So, if you are looking to protect some sort of confidential or private data, do not save the viz to Tableau Public. The level of protection provided by hiding a viz is very very minimal.