When light illuminates a pair of slits in a screen (top), the two overlapping wavefronts cooperate in some places and cancel out in between, producing an interference pattern. The pattern appears even when particles are shot toward the screen one by one (bottom), as if each particle passes through both slits at once, like a wave.
A demonstration towards the features within a wavelength. This diagram shows what are amplitudes, wavelengths, crests and trough. It shows that waves consist of these features and are presented in a mountain like presentation.