VirtualBox is a general-purpose full virtualizer for x86 hardware, targeted at server, desktop and embedded use. For a thorough introduction to virtualization and VirtualBox, please refer to the online version of the VirtualBox User Manual's first chapter.
VirtualBox is open-source software for virtualizing the x86 computing architecture. It acts as a hypervisor, creating a VM (virtual machine) in which the user can run another OS (operating system).
The operating system in which VirtualBox runs is called the "host" OS. The operating system running in the VM is called the "guest" OS. VirtualBox supports Windows, Linux, or macOS as its host OS.
When configuring a virtual machine, the user can specify how many CPU cores, and how much RAM and disk space should be devoted to the VM. When the VM is running, it can be "paused." System execution is frozen at that moment in time, and the user can resume using it later.