Error during update installaion? VM?? whatever that is

While installing the latest auto update (810030: Micrsoft VM Security
Update) just now it put this error:

"This setup will only upgrade over an existing version of VM"


Opaque to the point of crypticity or what -?? No indication of what VM
is - no suggestion of whether it matters or not - who writes these stupid
error messages ..... grumle ... grumble

Any way whatever it was it says history says the install was successful. I
don't know .. maybe its lying


What is this VM thing - is it good or bad - should I get it or be glad I
haven't got it

All advice gratefully received

Thanks

Peter