I didn't know what it was either, until I RTFA: > "This preference makes the browser associate all data (including cookies, HSTS data, cached images, and more)

I didn't know what it was either, until I RTFA: > "This preference makes the browser associate all data (including cookies, HSTS data, cached images, and more) for any third party domains with the domain in the address bar." They even included a link you can click on to get more info on it.