It is "root" in the sense of "root certificate"[0] which allows, potentially, the ability to MITM essentially any TLS connection transparently to the user.

[0] https://en.wikipedia.org/wiki/Root_certificate

"root access" has a different meaning than "root certificate"

Yes, I'm aware of that, which is why I "translated" for the article.

Any non-pinned connection, since some apps employ certificate pinning on iOS and can't be intercepted without modifying their application binary. This is uncommon, though.

A notable app that uses certificate pinning is Facebook.

Phew, I was worried that facebook would scrape my confidential communication with facebook.

We all know what "root access" means, and this is not it.

Agreed, I thought this must be Android then I saw Apple in the article and got really confused. I understood later when I got the root certificate but I've installed a number of those and would never call it "root access". This just allows them to MitM all traffic even TLS.

Ok, we changed the title to use representative language from the article, which is hopefully more accurate. (Submitted title was "Facebook pays teens to download research app with root access outside App Store".)

Indeed, for a second I thought Facebook had figured out a jailbreak and were paying people to try it out. That would certainly be very interesting news (and provoke even more divisiveness in the comments.)

Fortunately, the title has been edited now to remove "root access".