I've read online that "disk active time" means something like the amount of total time the disk spends on IO activity. I'm kind of confused about what that means. Also, on a virtual machine, what does disk active time mean?
"Disk active time" is the time spent reading from or writing to physical drives (hard drives, usb etc).
"IO activity" is transfer of data In or Out of the system, and usually implies some form of storage device is being used.
In the case of a virtual machine, it is exactly what it says on the tin, a virtual machine. This can include one or more virtual hard drives. A virtual machine can also directly access actual physical drives depending on how it is set up. As far as the software running within the virtual machine is concerned they all appear to be physical drives, and are treated and reported on as such.