tl;dr: Read the Quick Start Guide.
Drupal VM makes building local Drupal development environments quick and easy, and introduces developers to the wonderful world of Drupal development on local virtual machines (instead of crufty old MAMP/WAMP-based development).
It will install the following on an Ubuntu 16.04 (by default) linux VM:
It should take 5-10 minutes to build or rebuild the VM from scratch on a decent broadband connection.
Please read through the official documentation:
This VM includes many convenient utilities for local development, most of which can be enabled or disabled as you see fit (not everyone needs or wants an extra bundle of junk that they don't care about wasting their workstation's resources!).
Read the Drupal VM Documentation to learn more about how Drupal VM can fit in your development workflow.