A VM stands for a virtual machine. Virtual machines are a type of computing where you can host an entire computer within another computer. Technically, there are multiple ways of being able to achieve this. What it does is that it allows you to run a different computer environment within your existing computer. You can log in and have your desktop or whatever it may be that is entirely separate from your main computer.
Virtualisation or virtual machines are far more common in data centres and server farms. There is a host computer there that can run multiple virtual machines, with each machine having a different priority or purpose. People have slowly started to move away from hosting infrastructure and maintaining various virtual machines and are moving to cloud services where the virtual machines and virtualisation is abstracted and hidden from the user.