AirPods Pro Spatialize Stereo

Fixed means your head movement is not tracked.

The sound comes from straight in front of you.

Head tracked means the sound will change as you move your head or the phones position changes.

Fixed means that the audio follows your head movements. Spatialize stereo means the music is in a fixed space and when you move your head it stays where it is. You can check this out for more info: Listen with spatial audio for AirPods and Beats - Apple Support

Hope this helps!