It's not much different than what sites like MacRumors have done for decades.
MacRumors hosts some ads. The clickthroughs will generate ad revenue.
On MR, they might have content that espouses the awesomeness of WidgetX which can be purchased at Retailer A. MR gets a commission if you click through.
In the same way, some hot YouTuber or Instagrammer chick in a bikini will get a few dollars if you buy the detox tea she is hawking if you click through her link.
Here's a pre-smartphone version:
minus the hot chick in the bikini. Sorry about that.
This is really the electronic version of those deadtrees ads you get in your mailbox. Redeeming a paper coupon at a retailer kicks back some money to the ad agency that printed and mailed those ads. Or "mention coupon code IHEARDTHISONRADIOSTATIONBLAHBLAHBLAH for 10% off".
Did you ever buy something because someone you like said it was good? Same thing.
The main difference is that Millennials and Generation Z kids spend more time staring at their smartphones than the boob tube or carefully sifting through their snail mail detritus. The venue has shifted to social media sites (because that's where the audience is) but the basic concept remains the same.
Remember that the most desirable target group for advertising is the 25-39 age range. Today that audience lives on their phones. That's why newspapers and other print media are struggling mightily.