After a project has been marked complete on Malt, your client will receive a notification inviting them to leave you a rating (out of 5 stars) and a review. This is optional, but most clients tend to do this.
The rating associated with your profile is the average rating for all of the projects you've completed on Malt to date. It appears in the following places:
On your profile page.
Next to your profile "thumbnail" in Malt search results
In the conversation thread with your client in the Malt inbox
The better your rating, the more visibility you'll get in Malt search results. Not to mention, a strong rating gives potential new clients added confidence and peace of mind in your ability to tackle their projects successfully.