When your client opens your email containing the survey, we track that the images (your logo and the three faces) have been downloaded. We record this person has "seen" our survey. When the images aren't downloaded, we are only able to infer that the survey has been seen if the recipient clicks one of the smiley face images to provide feedback.
Response rate, is therefore, an approximation of how often your recipients see our smiley face survey, expressed as a percentage, and is calculated like this:
# of Reviews Received / # of Surveys Seen = Response Rate %
You can also read more about how to increase your response rate.