VirtualBox
In plain terms
A free, open-source desktop virtualization tool.
Going deeper
VirtualBox runs guest operating systems in virtual machines on your computer, useful for testing and isolation. It's a popular Type-2 hypervisor.
Under the hood
Runs atop a host OS; convenient for testing other OSes but slower than bare-metal or Type-1 hypervisors.
Examples
Running Linux on Windows, testing OSes, Vagrant backend, dev VMs.
Think of it like...
Like running a whole separate computer inside a window on your desktop.