All physics-calculations happen at fixed intervals – if they are calculated more often they are more accurate but more processing power will be needed. Generally it might be a good idea to adjust this to your target framerate. So if you’re targeting 25fps, you could set it to 0.04 (= 1/25). – This can be set under Edit > Project Settings > Time
If you set the physics timestamp lower than the framerat, physics objects might start to stutter. This can be fixed by setting the Rigidbody to Interpolate or Extrapolate in the Inspector. Unity will then calculate intermediate values for every frame where physics is not calculated based on the previous or next frame.