The optical board is set up with a mutli-slit baffle, a collimating lens, and a partially silvered mirror. Light from the point source passes through a slit baffle and is collimated into a series of parallel rays by a convex lens. When the rays hit the partially-silvered mirror, some of the light is reflected and some passes through the mirror.
This effect of partial reflection is often used as a bean splitter, splitting a single beam of light to follow two different paths. In this application, it may be used as a part of an interferometer, which measures very small changes in distance by looking at how the two light waves go in and out of phase with each other.