As @papertowels@mander.xyz said.

https://en.wikipedia.org/wiki/Active_noise_control

Historically, if you were in a noisy environment, you could get closed-back, circumaural headphones — headphones that fit around your ears and had a lot of sound-absorption padding — to help soak up the sound. I still use decent non-ANC circumaural headphones at home.

There are also some people who are more-willing to tolerate discomfort than I am who get in-ear buds, which block noise in their ear canal, and on top of that, fit ear protectors intended for industrial use, like 3M X5 Peltor ear protectors, which have even more passive sound absorption stuff than current circumaural headphones do, and are even larger.

That sort of thing works well on higher frequency sound, but not as well on low-frequency stuff, like engine noise, large fans, stuff like that.

ANC basically has microphones in your headphones, picks up on what sounds are showing up at your ear, and then tries to compute and play back a sound that produces destructive interference at your ear. That is, if you look at the sound waves, where the environmental sound is low pressure, it plays back high pressure signal, and when the environmental sound is high pressure, it plays back low pressure signal. It’s not perfect, or it could make environmental sound totally inaudible. But high-end ANC headphones are pretty impressive these days. I have a pair of Sennheiser Momentum 4 headphones — good, though not the best ANC out there in 2025, and I don’t personally recommend these for other reasons — and when they kick on, the headphones are designed to have the ANC fade in; same thing happens in reverse, fades out when you flip the ANC off. It sounds almost as if fans and the like around you are powering up and down when that happens, very eerie if you’ve never experienced it before. Even the sounds that it doesn’t do so well on, like people talking, it significantly reduces in volume.

And ANC does best with the other side of the spectrum, the side that passive sound absorption doesn’t — the low-frequency stuff, especially regular sounds like fans. So having both a lot of passive sound absorption and ANC on a given pair of headphones let the two work well together.

People often use cell phones in noisy environments, with a lot of people around, and ANC makes it a lot easier to hear music or whatever without background sound interfering. I think that it’s very likely that people will, long term, mostly wind up using headphones with ANC (short of moving to something more elaborate like a direct brain interface or something). It’s not really all that important if you’re in a quiet environment, and I don’t bother using ANC headphones on my desktop at home. But if you’re in random environments — waiting a grocery store line, in a restaurant with music playing over the restaurant’s speakers, on an airplane with the drone of the airplane engines, whatever — it really helps to reduce that background sound. ANC isn’t that new. I think that I remember it mostly being billed as useful for airplane engine noise back when, which they’re a good fit for. But it’s gotten considerably better over the years. For me, in 2025, good ANC is something that I really want to have for smartphone use.

The problem is that in order to do ANC, you need at least a microphone, preferably an array, and somewhere you need to have a model of the sound transmission through the headphones and be running signal processing on the input sound to generate that output sound. In theory, you could do it on an attached computer if you had a fast data interface, but in practice, ANC-capable headphones are sold as self-contained units that handle all that themselves. So you gotta power the little computer in the headphones. That means that you probably have batteries and at least for full size headphones (rather than earbuds) you might as well stick a USB interface on them to charge them, even if the user is using Bluetooth for wireless connectivity. And if you’ve done that, it isn’t much more circuitry to just let the headphones act as USB headphones, so in general, ANC headphones tend to also be USB-capable. My Momentum 4 headphones have all of Bluetooth, USB-C, and a traditional headphones interface, but…I just haven’t really wound up using the headphones interface if I have the other options available on a given device. Might be convenient if I were using some device that only had headphones output. shrugs