When two sound sources (for example tuning forks) emit waves of slightly different frequencies, one does not perceive separate tones, but a single tone whoose volume regularly decreases and increases. This phenomenon is called beating and can be explained by superposition of the two waves.
The first two diagrams of this app show the elongation (y1 respectively y2) as a function of time for two single waves of the same amplitude. The elongation y of the resulting wave for a given moment results from the sum of the individual elongations (y = y1 + y2). From the diagram below you can see how this elongation y depends on the time t. The observed variation in volume can be recognised as a periodic change of the amplitude.
The "Reset" button brings the simulation into the initial state. You can start or stop and continue the simulation by using the other button. If you choose the option "Slow motion", the animation will run 10× slower. The frequencies of the single waves can be varied between 100 Hz and 1000 Hz. (Don't forget to press the Enter key!)