Open Tracking gives you an estimate of how many people have opened your message. It is based on a small 1 pixel image we add to your message. This is why we call this metric Image Views.
It is important to know that adding this image will never impact your deliverability and that some email providers disable image display by default: Gmail for instance. If image display is disabled, we cannot tell you if a user has opened your email. This is for now the only technical option for us to get this information. It's the one implemented by all email delivery services.
Note that if a user opens your message several times, this might count towards your Image Views metrics.
Meaning of each term
All Image Views: We increment the counter each time an image is displayed (that is each time an email is opened). If a user opens an email several times, the "All Image Views" counter will always keep incrementing.
Image Views: We increment the counter when an image is displayed. However, if a user opens an email several times within an hour, we do not consider this action in our Image Views metric.
Image Views Rate: This is simply the percentage of Image Views compared to the total number of emails delivered by Critsend.