A simple ad system shows an ad in an app or page, and when you click on it, sends a request off to the ad server which causes some ad credit (money) to change hands - the advertised company gets charged for a click, intermediaries get a cut, and the app or site owner gets their share.
Unsurprisingly, a site owner is heavily incentivized to click on their own ads, or have others click on them, in order to pay themselves. To avoid this the ad server will want to check that the click came from a real human. They can check by IP (e.g. clicks coming from AWS boxes are probably fraud, excessively fast clicks are rejected, etc.); by user activity (monitoring mouse clicks, keyboard entries, dwell time, and a host of other factors); and by statistical measures (is the ad statistically likely to be clicked on by the source, based on e.g. language, prior ad preferences, etc.)
The news article makes it sound like the bot owners are gathering usage data to simulate real human behavior, which they can then pass off as being real human inputs prior to clicking an ad. Of course, one wonders why they don’t just instruct these millions of phones to click on ads directly (e.g. in the background), which would give them access to a huge legitimate pool of IP addresses.