I assume it's an img tag pointing at a one pixel image (every mail has its own querystring or URL), when you load it the server logs the time, requesting IP, and so on.
Obviously it'll only work if you tell the mail client to download the images.
(It doesn't need to be a single pixel, could be the bank's logo or whatever, but since it has to be unique to ID you and avoid caching and whatnot I assume making it only one pixel is more efficient.)