Just opening - or previewing - an email can display tracking images that can be used to collect all kinds of information about recipients, including:
when, where, with which email client, using which email provider and on what device the email was opened
how long the message was opened
how many times the message was reopened
whether or not you clicked any links within the message
what type of device you used to open the email
how many times and to whom the message was forwarded
Hackers, spammers, scammers can use this information to target you for phishing or other harmful activities.
A Simple Solution: Don't auto-display images
The simple solution is to disable the display of remote images or remote content. Even better, some providers can block all remote content. A few offer the ability to block embedded images, which can be used to transmit malware when displayed.
Privacy focused email providers like ProtonMail and Tutanota block external images by default. Tutanota doesn't even allow external images to be displayed by default, but it allows you to identify trusted senders so future emails can display images without prompting.
For now, the most effective way to protect yourself is to simply not display images in email. And watch out for similar techniques in texts and instant messages, too.