BitLocker is a great feature to have to keep your data safe, but if you don't partition your drive correctly before the OS install, you won't be able to install it, because you need an unencrypted partition to boot from. This tool will create another volume to boot from so that you can get BitLocker set up.
Previously this tool was only available by calling in on a KB article, but now Microsoft has made it freely available without the hassle of having to call and ask for it. You can get it here.