From the forum:
Update runs once per frame. FixedUpdate can run once, zero, or several
times per frame, depending on how many physics frames per second are
set in the time settings, and how fast/slow the framerate is.
Also refer to the answer given by duck in the same forum for a detailed explanation of the difference between the two.
It’s for this reason that FixedUpdate should be used when applying
forces, torques, or other physics-related functions – because you know
it will be executed exactly in sync with the physics engine itself.Whereas Update() can vary out of step with the physics engine, either
faster or slower, depending on how much of a load the graphics are
putting on the rendering engine at any given time, which – if used for
physics – would give correspondingly variant physical effects!