Listen Through With Transparency Mode
The other side of Active Noise Cancelation is Transparency Mode. With this option turned on, you can hear noises around you. This can be street noises to stay aware of what's happening, flight announcements or public transit announcements or it can just let you listen to someone nearby.
Another use case is when you need to briefly respond to someone. Instead of taking your AirPods Pro out, or removing one, which will stop your music, simply turn on Transparency and chat, then get back to listening immediately.
You squeeze one of the stalks to enable this and then squeeze again to turn it off. You can also control it in the Control Center on your iPhone or Apple Watch.