It's a tracking tool. You have a bunch of sites embed an image, and requests to those sites also make requests to said image, which you can use to start tracking a client. A single pixel is merely the cheapest image.
I recall Facebook doing it years ago, I imagine they still do.
A 'pixel' is an unobtrusive (as in, not seen by the user like a banner ad is seen) asset* served on a web page that can cause the user's user agent to make an affirmative web request from you, a third party, so you know someone was at the site serving your pixel.
Typically used for:
- tracking in general, as well as more specifically:
- retargeting
- conversion
* Note: Doesn't have to be a literal pixel, but a literal transparent pixel is least likely to get blocked. Serve your pixels from the end of a parameterized path (/some/param/or/other/pixel.gif) and it's not seen as query string tracking either.
I recall Facebook doing it years ago, I imagine they still do.