An outward-facing microphone detects external sound.
AirPods Pro then counter it with equal anti-noise, cancelling the external sound before you hear it.
An inward-facing microphone listens inside your ear for unwanted sound, which is also eliminated with anti-noise.
Noise cancellation is continuously adjusted at 200 times per second for truly immersive sound, so you’re fully tuned in to your music, podcasts, and calls.