The term hologram is more of an analogy than anything else, you shouldn't think of it as a projection produced by light. The idea is that we're living on the edge of a high-dimensional membrane and that our 3 dimensions (+ time) form the surface of that membrane. One of the interesting thing about membranes is that the surface encodes a lot of information about the interior structure, or stated differently, the interior determines the surface. That's what they mean by hologram.

The other replies here reference a 2D encoding of a 3D space. That's a simple example of this kind of holograpy but it's not the one being tested for. Most of hte holographic theories reference a much higher dimensional space on the inside and a 3 dimensional surface.