In quantum mechanics, the concept of matter waves or de Broglie waves /d??br??/ reflects the wave–particle duality of matter. The theory was proposed by Louis de Broglie in 1924 in his PhD thesis. The de Broglie relations show that the wavelength is inversely proportional to the momentum of a particle and it is therefore called the de Broglie wavelength.
? is a wave function (or the where abouts of an electron in space). This example words best with shrodinger wave function graphs for a simple diatomic wave function of hydrogen gas. (just check it out on google images). Basically, the way to read the graphs, is you will see two points on the graph. If it is constructive interference, both points will be above the x axis. These two points, represent the nuclei of each hydrogen atom in respect to one another. When you square this wave function i.e. "?2", you will notice that the graph becomes sharper.