It is the root password of your OpenSUSE Linux - your BIOS needs permission to install it - kind of like what sudo does when in a terminal.

If you install virtualbox via "sudo zypper virtualbox" it is signed with the OpenSUSE MOK. If you install it from Oracle VirtualBox - you have to sign it yourself and that is the key the BIOS is looking for to install virtualbox-kmp-default.

semi offtopic question here additionally about mokutil and its
database and this password one sets when adding? enrolling? settingup?
another key into its database.

what is this password exactly about? is this a kind of one time
challenge/password just to be asked once during next reboot to insert
the additional key into the mok database? why? (maybe it can?) can it
not be inserted directly into the MOK-database directly? is this some
security measure? how? did enterprise suse corporate key become
inserted into the MOK database? what was the password for that
situation? can i just have or re use that password to begin with? i
kind of like to know what reasoning there is behind a password
protection and what architecture. can or must i keep track of this
password (my password) for the future? or is it only like used once
only (as i have thought about it above)

i dont yet understand the password concept of this MOK database stuff.
i found some whateverxxxxexchange discussion about some debian or
ubuntu folks also trying to virtualbox installation their stuff and
having huge loads of problems with the MOK password about early boot
initial keyboard layout settings and what not and even such claims as
MOK password were only to be allowed for five characters max or stuff.
this kind of hints for me that its rather a one time only challenge or
something.

anyone? ty