* Beats are a phenomenon where two sound waves with slightly different frequencies interfere, creating a periodic variation in loudness.

* Reflection is the bouncing of waves off a surface.

* Refraction is the bending of waves as they pass from one medium to another.

* Diffraction is the spreading of waves as they pass through an opening or around an obstacle.

Interference is the key to beats because it involves the superposition of waves. When two waves with similar frequencies interfere, they can reinforce each other (constructive interference) or cancel each other out (destructive interference). This alternating pattern of reinforcement and cancellation creates the periodic loudness variations we hear as beats.