From the FAQ:
What is a privacyscore?
A privacyscore is a way to assess the privacy risk of using a website.
Privacy risk is the chance that data about you will be used or shared in ways that you probably don’t expect. Privacyscores cover two kinds of data:
We estimate privacy risk to personal data (such as your name or email address) based on the published policies of the website.
We estimate privacy risk to anonymous data (such as your interests and preferences) based on the privacy qualifications of the other companies who collect this kind of data across websites.
You can see privacyscores of the sites as you visit by using the privacyscore add-on for Firefox and Chrome.
Read more on Privacyscore.com