Use this function to make your game frame rate independent.
If you add or subtract to a value every frame chances are you should multiply with Time.deltaTime. When you multiply with Time.deltaTime you essentially express: I want to move this object 10 meters per second instead of 10 meters per frame.
When called from inside MonoBehaviour's FixedUpdate, returns the fixed framerate delta time.
Note that you should not rely on Time.deltaTime from inside OnGUI since OnGUI can be called multiple times per frame and deltaTime would hold the same value each call, until next frame where it would be updated again.