Users can vote for the grades of each track. The displayed grade is an average of the grades all users have given. To be able to grade tracks, you must be logged in. Before you can login, you must create a login for yourself.
You can grade tracks from 1 (worst) to 10 (best). The grade should reflect just simply how much you enjoy racing the track. To grade, log in and click on the "Comments" link on the track you want to grade. There, in the track information is a pop-up "Your grade". Select the grade you want, and press "Vote!" button.
The vote you make is not a final one, you can change it later if you find your opinion about the track changing. And while you are at the comments page, you can leave more detailed comment about the track too.
A general rule of thumb about the grades would be:
grade 1 - you hate the track
grade 5 - it's ok, you don't mind racing it nor you enjoy racing it
grade 10 - you love racing the track
And everything else falls somewhere between.