Play two sine tones with nearly the same frequency and listen to the slow rise and fall of loudness: the beats. Each slider sets the frequency of one tone, and the beat frequency is the difference between them. Press Start audio once; the sound keeps playing while you move the sliders.
What to notice: the two tones are plotted over a short window where you can see the individual cycles; the sum is plotted over a longer window so the beat envelope is visible. Count the beats per second and check that the number matches |f2 − f1| = Hz.