The Concealed Clipboard feature stops data that you copy from the Strongbox app from being recorded by clipboard manager apps, like Maccy and Paste.
When Concealed Clipboard is enabled, Strongbox tells clipboard apps that what it has copied to the clipboard is sensitive and should be concealed, so that these apps don't display or record your passwords/credentials etc.
Please note that this does not guarantee that your clipboard manager app will comply with the request.
Concealed Clipboard can be turned on and off in the Advanced app settings.