Huygens' principle states that each point on a wavefront can be seen as a source of secondary wavelets. The interference of these wavelets determines the shape of subsequent wavefronts as the wave propagates. Interference occurs when two waves superimpose, resulting in areas of constructive and destructive interference where wave amplitudes add or cancel. Constructive interference occurs when the path difference between waves is an integer multiple of the wavelength, while destructive interference requires an odd integer multiple. Beats are heard when tones of similar frequencies interfere, with the beat frequency equal to the difference between the frequencies.