It's shortcut(blame google for that) for network system settings which allows you change 2G-3G and data roaming mode with 2 clicks instead of 4
How to add widget:
If you on HC, ICS or Jelly Bean:
Open All Apps -> Select Widgets tab -> Find widget in the list
For older versions:
Long click on empty space on your home screen -> Widgets -> Select widget from the list.
If widget doesn't appear on the widget list:
it is a bug in launcher, try to restart it:
Settings -> Apps -> Running tab -> Click on Show Cached Processes -> Select Launcher From the list and click Stop.
Try to re add widget
NOT FOR CDMA PHONES
Check our OnOff Widgets Pack!