When a campaign is sent, MailChimp embeds a tiny, invisible graphic to the bottom of your HTML email. This "open tracker" graphic, or "web beacon", is unique to each campaign you send.
Every time someone opens your email, that open-tracker graphic is downloaded from our server. It's what we use to track "opens".
Open tracking is selected by default on all email campaigns. But it can be turned off on the Setup step of the Campaign Builder. 
So what's that mean?
- You can't embed graphics into plain-text emails. So open tracking only works for people who view your HTML email. That's why the "track opens" option is only available under the HTML email section.
- We can only track opens if your recipients have "View Images" turned on or if they choose to "show images" when they view your newsletter. Some email applications automatically block images from displaying by default, especially if it's from someone that's not in their address book. So unless the recipient clicks their button to "display images in this message", we won't be able to track their open.
- When someone signs up for your list, you should ask them to add your email address to their address book. That way, your images will more likely be "on" by default.
- If you work for a financial institution (or some large company or university), check your privacy policy. You may not be allowed to use "web beacons" in your messages.