The company shares your online activity with advertisers. It uses something called a supercookie to do it. (Read more about it here.) It attaches a unique identifying header to your Web traffic.
The idea is that advertisers can follow you around, to see which sites you're visiting. Then, they'll match your interests with specific ads you might like. If you remember, AT&T used to use supercookies but stopped after receiving a lot of backlash for invading your privacy.
Note: Read these tips to stop carriers from tracking your mobile browsing. Plus, keep reading Happening Now for updates on the FCC's March 31 vote on its proposed new privacy rules.