Virtual Box is a software, that allows to emulate a PC within your PC. It is extremly useful for development, e.g. having Mac OSX and then get it Linux or OPenBSD installed inside Virtual Box.
Inside this operating system you can then play with many different tools or wallets, and best thing is you can have several virtual machines in parallel, so you can even setup your own little data center, to verify communication between systems.
So yes, very useful for bitcoin development.
More info on virtul box here:
https://www.virtualbox.org/