What does 3DoF mean for smart glasses?

Rotation is only part of head movement

We find a chair useful for explaining the distinction. You can turn your head while remaining seated in the same place. You can also lean sideways or move the chair without turning your head. A system that tracks rotation handles the first kind of change; translation needs additional position information.

For a demonstration, turn toward a virtual screen, then lean closer without changing your viewing direction. Observe whether the screen responds to the change in position. A screen that behaves well while you rotate has not thereby demonstrated six-degree-of-freedom tracking.