TL;DR: There are ultrasonic waves embedded in advertisements (e.g YouTube ads, TV Commercials) that bearly can be heard. Though some apps are constantly waiting for those waves, to track you. Don't approve microphone access for apps before you ask yourselves "Why would they want to use my mic?"
Ultrasonic ads are a fairly new species of privacy-invading advertising that no much people know about, but if you care about your privacy, it should concern you.
How does it work?
High-frequency tones that probably won't be picked up by a regular human ear are embedded into advertisements that use audio. Those unheard tones contain metadata about the ad itself and are targeted towards devices that are listening constantly, like your smartphone, or other "Smart House" equipment such as Amazon's Alexa or Google Home. Once these tones were picked by one of these smart devices, they are probably sent to an advertiser server, where they're analyzed in order to get as much data as they can about you, a consumer, while of course, invading your privacy.
This technology is still in its infancy. There are some advertisers that use this technology to send location-related ads, such as tickets for shows or local hotel deals, yet it might be more frightening, as explained in this imaginary example: I want to buy a new pair of jeans, so I'd go to my favorite mall, where all clothes are expensive. At the mall, there are ultrasonic beacons that always transmit those tones and my phone picks them up. The advertiser now knows that I probably spend more money on jeans than other people around me, so he might not offer me jeans at a discounted price in the future. A lot could be inferred from this - where have I been, what I probably did and what I'm up to. SCARY.
Lately, researchers have shown a method of exposing Tor users using ultrasonic beacons.
How do I know if my phone listens to these tones?
Well, in most cases, by default, none of your installed apps should be listening to ultrasonic tones, though apps you might've downloaded should. Most of those apps use the Silverpush SDK or similar to track tones. And according to some articles, there are over 200 apps that are using this SDK, this paper says including McDonald's'. It's hard to know if an app is listening to you secretly at a first glance, but if it asks for your permission for microphone access, it should ring some bells.
Those companies are probably counting on users to just allow the requested access without any questions. Be alert!
For your computer, I recommend installing an adblocker extension for your browser. I use the TunnelBear Blocker, it's free, light on the CPU and has a fun and explanatory graphical interface! Oh and it blocks all other annoying ads, including ultrasonic ads.
To sum it up, advertisements are getting smarter every day, which comes at the expense of our privacy as individuals, we should be smarter and take nothing without thinking about its rational. I hope that you're a bit educated and familliar with those type of ads, so don't let any app fool you!