a VM is a virtual machine. In simple words, it is virtual hardware(emulator) to run your virtual devices.
from Wikipedia:
In computing, a virtual machine (VM) is an emulation of a particular
computer system. Virtual machines operate based on the computer
architecture and functions of a real or hypothetical computer, and
their implementations may involve specialized hardware, software, or a
combination of both.
one more reference
A virtual machine (VM) is an operating system OS or application
environment that is installed on software which imitates dedicated
hardware. The end user has the same experience on a virtual machine as
they would have on dedicated hardware.
Virtual box is tool provided by Oracle to create virtual machines:
Oracle VM VirtualBox (formerly Sun VirtualBox, Sun xVM VirtualBox and
Innotek VirtualBox) is a hypervisor for x86 computers from Oracle
Corporation. Developed initially by Innotek GmbH, it was acquired by
Sun Microsystems in 2008 which was in turn acquired by Oracle in 2010.VirtualBox may be installed on a number of host operating systems,
including: Linux, OS X, Windows Vista, Solaris, and OpenSolaris. There
are also ports to FreeBSD and Genode.It supports the creation and management of guest virtual machines
running versions and derivations of Windows, Linux, BSD, OS/2,
Solaris, Haiku, OSx86 and others, and limited virtualization of OS
X guests on Apple hardware.
and finally they are used to emulate the virtual devices you need.
solved What is vm in virtualBox used for [closed]