When your client opens your email containing the CSAT 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 CSAT Reviews Received / # of Surveys Seen = Response Rate %
You can see this in the box showing response rate on the reviews page:
The percentage is the response rate, and the numbers underneath show you the actual count of reviews and surveys seen for the selected time period.
If you want to get more reviews, have a look at how to increase your response rate.