Vagrant is a tool that allows the easy creation of virtual machines. It was originally developed for use with VirtualBox, but it has been extended to allow integration with other virtualisation tools. Using Vagrant you can create a particular setup that you can then share with other people without having to give them large virtual disk images.
I have to admit, when I first heard of Vagrant I didn't 'get it' and wasn't sure why I should be using it at all. Since then I have done lots of reading and talking with other developers and I am now wondering how I managed to work without it.