Find two people who can sing in a high falsetto voice. Have the first person sing a high "ooooooh" note for several seconds. The second person should stand very close to the first singer (face-to-face, or even cheek-to-cheek), then sing and match the same note. One of the singers should then sing slightly higher than the other, purposefully out of tune. Suddenly, besides the two notes, you hear another unusual sound, almost a rattling sound, a very quick louder-softer-louder-softer pattern.
Sound is a wave phenomenon. In the case of the singers, the sound wave travels through the air. When the two singers sing slightly out of tune, the sound waves from the first singer overlap with the sound wave from the other singer. This overlap is called "interference". The strange "rattling" sound is the result of this interference. Those rattle noises are known as "beats". You can also hear beats by plucking two guitar strings that are slightly out of tune. Again, you will hear it as a pattern of louder-softer-louder-softer.