Transparent single pixel images hidden on a web page (or in an email with html) that load from a tracking server. They log your IP address and other info when you load the webpage or open an email. This data is aggregated and used to "fingerprint" users based on web browser/version, OS/version, installed extensions etc.

You can block them by using a decent adblocker extension like uBlock Origin or a browser that has built-in adblocking like Brave Browser. For email, check for a setting to "ask to load images" or something like that. I have images disabled in Gmail for this reason. You get a button to load the images on an email by email basis if you decide it's from someone you trust.