Use Guest as an authentication method when doing a Remote Virtualbox connection?

Question

Currently I got virtualbox working with win10 remote desktop application with the authentication as NULL in virtualbox for my ubuntu VM. I tried to use "guest" as the authentication method but win10 remote desktop app gives this error:

[Window Title]
Remote Desktop Connection

[Content]
This computer can't connect to the remote computer.

Try connecting again. If the problem continues, contact the owner of the remote computer or your network administrator.

[OK] [Help]

What does guest authentication mean exactly? Is it the user/pass of the ubuntu vm or my window 10 host computer?

I just don't feel secure with a "null" authentication method for doing a remote vm as I work in a office with other people.