The 6000 are milliseconds and half of it are 3000 milliseconds, so 3 seconds, so we switch every 3 seconds between the small and big rect.
Actually, it doesn’t have to do with the 60 FPS.
Since we use the millis() it would work with 30, 40, 60 or 100 FPS.
In fact you can try this by setting the frame rate to 33 or so: command frameRate(33);