See: Mobile Advertising.
Ads on a cell phone. You know those annoying things Shmoop puts up there when you look at a page of ours? Yeah, they pay the rent, tyvm.
Mobile marketing isn't just ads on cell phones, though. The notion includes things like billboards and other GSP-Big-Brother-like trackers that follow you wherever you go. The radio is kinda mobile marketing, at least if you listen to it (at all) when you're out and about. The whole notion of, say, a local restaurant knowing that you're within half a mile at 11:22 am and being able to ping you about three bucks off on their Boofaleau Burger Special? Yeah, it's powerful stuff commercially.