Your computer's main operating system is Windows, but it has a 'virtual machine' running Linux Ubuntu.
What is a virtual machine? This is basically a simulation of a computer. The software 'VirtualBox' can simulate different operating systems. We're using a virtual machine to provide a Linux environment in which to perform the work required for this unit.
What is Linux Ubuntu? Linux is a basic operating system, but you can add various configurations or collections of software to give it different behaviours etc. These are then released as 'distributions'. There are many different Linux distributions, but Ubuntu is the most popular due to its relative similarity to Windows.
This guide will show you how to log on to the Linux virtual machine.