In essence, virtualization is about sharing one set of physical resources, like a CPU, HDD, RAM, etc. between multiple virtual systems. One physical system is essentially split up to make several virtual systems that share the physical resources in some way. Each virtual system is isolated from the other systems to some degree, but this depends on the type of virtualization used.
virtualization is a very powerful technology that has helped enable the entire concept of Cloud computing that we know and use today. You'll have used virtualization if you've ever created an AWS EC2 Instance or another kind of Virtual Private Server (VPS) such as a DigitalOcean "Droplet". These are based on virtualization technologies.
This will be enough for you understand what is going on under the hood. You don't really need to know more than that at this point in time.