The monochord is the oldest physics apparatus we know — one stretched string over a movable bridge, on which Pythagoras found that pleasing musical intervals come from whole-number ratios of string length.
A plucked string rings as a mix of standing waves: only patterns whose half-wavelength fits the string a whole number of times survive, at frequencies fₙ = n·v / 2L.
Slide the bridge and every natural frequency shifts with it — half the length is twice the frequency, which our ears call an octave.
The bow drives the string at one steady frequency. Park it on any fₙ and the amplitude grows enormously: resonance. Motion here is slowed 100× so you can watch the wave; the frequencies read out true.
Something in the simulation stopped unexpectedly — the lesson continues without it. You can move on; nothing you did was wrong.