The rating system shows the amount of trust other users have in your counterparty. It indicates whether he has been fulfilling his obligations to other parties, whether he has been making fast payments, is responsive, etc. With a public rating and reputation, users are incentivized to continue being a suitable counterparty in the future.
The rating shows the amount of positive and negative reviews given to the user by others.
You can rate your counterparty after successfully completing a contract.