Ratings represent how well other GrabMe users think you have done both delivering, and requesting. This is a simple star rating that goes from 1 to 5. Where 1 is pretty bad, and 5 is pretty amazing.
Ratings are important because they give other users a quick snapshot of what others think of their interactions with you. More importantly, they help build a since of trust. The fact is, there are plenty of bad things that can happen when you show up at some strangers house, or delivery location. Ratings help assure users they can trust you.
They are calculated by simply tallying the number of stars you have received out the number you were eligible to receive. So if you make three deliveries and you get 4 stars for each one, you have an average of a 4 start rating.
Finally, ratings are not applied to your profile until both the deliverer and the requester have rated the transaction*. However, you cannot complete a transaction, without leaving a rating.
*Note: it can take a few minutes after a request has been completed for the new ratings to appear on your profile.