An email tracking pixel is a tiny 1x1 transparent image embedded in an email. When the recipient opens the message, their email client loads the image, and the open is recorded with the time and approximate device info.

The easiest free option I've found is MailPixel, a free email tracking pixel generator that needs no signup. It also has a Gmail Chrome extension that adds tracking to every email you send.

Step-by-step guide: how to use a tracking pixel.