Are you refering to de Broglie's matter waves? As Hawking said, physics' job is to ask how. It's philosophy's business to ask why. But briefly, interference effects can be observed with electron beams, but not with marbles, and the shorter the wavelength of anything, the more it acts like a particle.
h is a fundamental constant of nature and [tex] p = \frac{m \, v}{\sqrt{1 - v^{2}/c^{2}}} [/tex] is the momentum of a particle with mass m moving at speed v and c is the limit speed with which any interaction can be transmitted and is another fundamental constant of nature.