A Shepard tone is a stack of sine components exactly one octave apart, played under a fixed loudness envelope: components fade in at the bottom, pass through a loud middle region, and fade out at the top. When the whole stack glides slowly, the pitch seems to move for ever without arriving anywhere. Press Start audio and let it run for a while.
What to notice: the tone always seems to rise (or fall) yet never gets anywhere. Watch the dots on the curve: each component quietly leaves at one end and re-enters at the other, so the stack as a whole never changes register.