Spatial audio is only avaliable while using AirPods Pro or AirPods Max with supported iPhones and iPads (no doubt it will arrive to other Apple devices sometime in the future also).
The intention of the feature is to increase the sense of immersion by using surround sound, making it sound like audio is coming from your device (even if you turn your head) or even in some cases the audio may appear to be coming from behind you, or to the side.
Apple has described spatial audio:
Spatial audio with dynamic head tracking brings the movie theater experience right to your AirPods Pro. By applying directional audio filters and subtly adjusting the frequencies each ear receives, spatial audio can place sounds virtually anywhere in space, creating an immersive surround sound experience. Using the gyroscope and accelerometer in your AirPods Pro and your iPhone, spatial audio tracks the motion of your head as well as your device, compares the motion data, and then remaps the sound field so that it stays anchored to your device even as your head moves.
Spatial audio is also something which has recently became available on Apple Music, with stereo Dolby Atmos songs supporting it. It works in a similar way, with Apple describing it as a “multi-dimensional experience”.
Netflix is only just rolling out support for this feature, a little while after it launched. It should be avaliable for everyone over the coming days. The feature has already been avaliable on Apple TV+ for a while, where it appears to work really well.