A tinsmith starts with a flat square of tin, snips a small square from each corner, and folds the four flaps up into an open box. Cut too little and the box is a wide, shallow tray; cut too much and it's a tall, skinny cup. Somewhere between sits the roomiest box the sheet can make.
Its volume is exactly V(x) = x·(L − 2x)². As you drag a corner, the amber fill swells and shrinks — and the little side chart plots that whole curve.
The needle reads the slope of that curve, dV/dx. Where the box is at its fullest, the curve is momentarily flat, so the needle points straight up at zero. "Set the derivative to zero" is just the mathematician's name for the flat top you can feel with your hands.
Solve dV/dx = (L − 2x)(L − 6x) = 0 and the best cut is x = L/6 every time — a result the same for a matchbox or a shipping container.
Something in the simulation stopped unexpectedly — the lesson continues without it. Nothing you did was wrong; you can move on.