It updates the /etc/alternatives links, which defines default applications for the traditional Unix/Linux system, for example which java virtual machine should be used in case there are several versions installed on the system, which webbrowser will be launched etc.
Basically, it is a graphical front-end to the update-alternatives command-line tool.
Install on Ubuntu
sudo aptitude install galternatives
Finally, we could create a launcher in our main menu (System-->Preferences-->Main Menu):