Mr. Zak is right that you'll get better answers on the 3dPrinting.SE, but the answer is that it will print a solid cube. Printing starts at one face, and continues until it finds another face (sort of). So if you only have one face, it knows where the material starts, but it doesn't know where the material stops. If you wanted a hollow cube, you'd have to specify a width for those walls by creating an inner cube with normals pointing toward the center of the object. The printer can't (won't) assume how thick a zero-thickness polygon should be.
It's useful to remember that all 3D polygons only have 1 face. This can be confusing, because there's nothing in the real world that works this way. That's why a single face is considered "non-manifold."
Duarte's answer includes the same information, but the terminology might not be familiar to you. A closed, coherent, manifold shape is one that is:
- closed: all edges of all faces are shared by 1 and only 1 other face, like the default cube.
- coherent: (essentially) it that has dimensions that can exist in the real world. Intersecting geometry, or geometry that has zero width is not coherent.
- manifold: (basically) geometry that has one distinct "inside" and one distinct "outside." More specifically, all the normals are pointing the same direction. With the default cube, you have normals on the outside, and they are all pointing outside.
If you were to do this:
You would have an interior face, with a normal that is neither pointing inside... nor pointing outside. Thus, this is non-manifold geometry.
For a more thorough a technical discussion of non-manifold geometry, see the answer to this question.