The interval in seconds of in-game time at which physics and other fixed frame rate updates (like MonoBehaviour's FixedUpdate) are performed.
The fixedDeltaTime interval is always relative to the in-game time which Time.timeScale affects. A fixedDeltatime of 1 second in a game with a Time.timeScale of 0.5 means fixed updates occur every 2 seconds of real time.
See Time and Frame Rate Management in the User Manual for more information about how this property relates to the other Time properties.