Usually, on a computer, you use only one Operating System (OS) like Windows for example. Even if you can install several OSes on a disk, you usually only use one at a time.
Virtualization allows you to run several OSes at the same time.
Basically, this is how it works:
- your main OS runs like usual (Windows in LiLi's case). This OS is called "Host OS"
- a virtualization software will launch a second OS on top of the first one
- the virtualization software (VirtualBox in LiLi's case) will trick the second OS and give him some virtual hardware.
Each system is not aware of the other's existence. For Windows, the virtualized Linux is just a software running (VirtualBox). For Linux, it thinks it's running like usual except that it is not. It is emulated by a software (VirtualBox).