The message open pixel is a tiny, invisible image that resides on our servers. When a recipient opens your email, the image is downloaded, and we're able to record that download as an open for the campaign.
It's important to know that message open statistics are not 100% accurate. Recording an 'open' only takes place when the recipient's email client (e.g. Outlook) is both capable of displaying HTML and actually downloads the images contained within your email (including our message open pixel).
This means that if you're sending a text-only email, Nex.to isn't able to track opens. The same holds true for recipients reading your HTML email without images showing.
The only exception is that we do record an open for people who clicked a link in your campaign, even if they didn't download images, since we obviously know they read at least some of your email.
Another issue is that your recipients may have a preview pane in their email client. That preview pane might display your email automatically (and therefore download the message open pixel) without the recipient ever having to click on it or read it.
So, message open statistics aren't hard and fast numbers. And because you can never know the true figure, it's better to use these as a general guide and as a way of measuring the trends among your email marketing campaigns.