Templates Insights is a BETA feature that offers data-backed suggestions to improve the predicted reply rates of your email Templates. You can learn more about the algorithm here. Please note, this is only available on Gmail.
To estimate how effective your Template is, click on the "Template Insights" button in the lower right of the Templates editor. This will expose the Templates Insights calculator. Here, you will be able to view your Template grade and determine what could be improved upon. Click on each category to learn more information about what makes a great email.