Like a piece of paper, surfaces in 3D have a front and a back side. However with 3D models usually only one side is visible for performance reasons, this is called culling. Unless stated by the author, a piece of paper in software would look normal from one side, and be invisible (culled) from the other, like shown below.

All 3D surfaces have a main side (the front face), which can be decided when authoring the model, but software can separately choose to show things differently. Like showing both sides, or flipping the order and only showing the back side (see visualization under). Having both sides visible however is usually a waste of computing power, as most models are enclosed and you only ever see one of the sides. Though a few cad/architecture softwares does assume both sides are visible, and would appear to be "missing" parts in our software without the correct settings. In cases like these you should turn off the "optimize faces" setting before starting your room.

Here you can see a more thorough demonstration of the three face culling options.