Change the frequency of Oscillator 1 and Oscillator 2 to demonstrate what are referred to as "beats"

"Beats" occur due to the Superposition Princle. In musical terms, if two violins are playing a note such as A 440 and they are perfectly in tune, their collective loudness is the sum of the loundness of viloin 1 and the loudness of violin 2. If the two violins are slightly out of tune as demonstrated by the two oscillators above set to 440.00 Hz and 440.50 Hz, you will hear the the collective loudness of the two oscillators become silent for a brief moment.

A quick explanation of the Superposition Principle is that sound waves "add", and that is referred to as "constructive interference" and "destructive interference" depending on their phase relationship. The graphic below shows two sine waves deomstrating "constructive interference" on the left side of the graphic and "destructive interference" on the right side of the graphic. The right side of the graphic shows two sine waves 180° out of phase, and when their amplitudes are added together they cancel each other out exhibiting 100% destructive interference. The left side of the graphic shows two sine waves 0° out of phase, their amplitudes adding together exhibiting 100% constructive interference; in musical terms they are perfectly "in tune".

When you hear beats between two sound sources, such as two violins or tumpets, you are hearing the sum of their collective amplitudes exhibit "constructive interference" and "destructive interference" due to the Superposition Principle.