An appeal from the browser to the website to not track the user's surfing habits. Firefox was the first browser to include a do-not-track option for the user, and the others followed. However, the reason this is an "appeal" is because it relies on the cooperation of tracking companies that are self-regulating and offer an opt-out function. However, most of the large ad networks in the U.S. are compliant.
The "do-not-track=yes" is a header in the HTTP request for a Web page that is sent to the website. See tracking cookie
and Web bug
Firefox Privacy Setting
Clicking this box sends a DNT header with all Web page requests. Firefox was the first Web browser to offer this, and the others followed.