The Globe At Night website provides all of the details, but in summary you compare what you see in your sky (your backyard, rooftop or any other convenient observing location) to a series of images on the Globe At Night website. By finding the image that best matches your view and reporting this on the website, you are helping to gather data from cities around the world on the relative light pollution in the sky. The constellation Orion contains stars of varying brightness, and depending upon the darkness of your skies, you will see more or less of these stars. The charts on the Globe At Night website show you Orion with differing "magnitudes" of stars, so for very light polluted cities you might only see the 1st and 2nd magnitude stars, whereas in darker skies you will see 3rd, 4th, or even 5th and 6th magnitude stars (the higher the number, the fainter the star).
Join the worldwide "Citizen Scientists" supporting this effort to increase awareness of the night sky, and take part. And bring along a friend. It's fun!