Two coherent sources ripple across a tank. Bright bands are constructive interference; dark bands are destructive.
When two waves meet, their amplitudes add. In-phase → brighter (constructive). Out-of-phase → cancellation (destructive).
Shorten the wavelength or widen the source separation and the interference fringes get finer. This is the same physics as Young's double-slit experiment that first showed light behaves as a wave.