Readers: In general these are people who have opened your email. In specific, to be counted as a "reader" the images in your email need to have displayed. For many email software clients this requires the recipient to click on a button to display images. If they don't they'll still be able to read text and click, but because the images have not displayed, they won't be counted as a "reader."
Throttled: These are emails that have not yet made it to the readers account. It could be because they have not yet been sent or because the ISP has not yet allowed it through. To achieve strong sender reputation (which helps your email delivery) on our end we throttle the first email to a recipient over a 7 day period. Subsequent emails get throttled over a 24 hour period to those who have not yet read an email of your or are sent immediately to those that have previously read an email from you.
Visitors: These are people who have clicked a link in your email. This does not include those that have clicked the "Unsubscribe" link.