Clicking the icon shows you all the possible brightness values.
The current value is indicated with a dot.
Clicking a menu item sets the brightness to the desired value.
Scroll wheel event is enabled on the icon. Scrolling up means lower brightness, scrolling down higher.
Install Brightness indicator on ubuntu
Download deb package from here and install by double clicking on it.
After installing the package, add the program to your startup programs:
Run System Settings » Startup Applications » ‘Add’