How does Mailchimp contact rating work?

How contact ratings work

Mailchimp uses a 16-point scale to measure contact engagement. We convert that number into a star rating to help you understand how a contact rates among others. Every contact starts with a zero engagement score, which is a 2-star rating.

To determine where contacts fall on the engagement scale, we measure their click activity against your send frequency. We recalculate contact ratings each time you send an email. It can help to think of this in terms of averages. For example, we'd rate a contact who doesn't click a link in a monthly email higher than one who doesn't click a link in a weekly email.

You can learn more here - https://mailchimp.com/help/about-contact-ratings/#How_contact_ratings_work