Computer virtualization is where computers run inside other computers.
With computer virtualization your computer could be running Windows 10 as itsí operating system and you could create a virtual computer in it that runs Linux. Both machines can be running at the same time. In the past when we did dual booting you could one OS or the other but not both at the same time.
With virtualization you create a computer inside the running computer. The virtual computer runs just like a real one, just there is no real computer. You install operating systems (Windows, Linux, etc.) in it and then applications in the virtual machine like on a real machine. You can turn the virtual machine off or on, put it to sleep, and it has hard drives (which are actual files on the real disk of the real computer).
Virtual machines work very efficiently and with the memory and hard drive sizes and cheapness, you find in many businesses now that where they used to have a number of servers they now have a lot fewer physical servers (saves on power and AC) and the additional servers they had still work the same as separate but are now virtual machines on the few physical machine.
Popular virtualization programs are Virtual Box by Oracle, VMWare and Microsoft Hyper-V. Virtual Box is available free on the Internet and the others you must pay for to use, although VMWare Player which is their bottom product is free to certain groups.