Hi McGuyver (hm... I seem to have been beaten but anyways...)
This is not a stupid question at all and you guessed right; it's especially relevant if you want to export your model to other applications (I know it causes problems in 3ds format for sure).
First of all, you need to understand that since in "real world", everything has some kind of thickness, therefore virtual, 3D models should also have thickness. Then there is a difference between "solid modelers" (where this "thicknes" is symbolised by some kind of a solid object) and "surface modelers" (like SU) where we only "pretend" this thickness by creating hollow boxes.
Now the faces in these latter modlers obviously have two sides; a "front" (that should always be oriented to the "outsie world") and the back (which should be orinted towards the "inside thickness"). Normally only front faces are supposed to be displayed to the viewer (as you never go inside the thickness of anything unless you are a ghost going through walls).
In SU, the "default" front face (depending on the template you use) is whitish or light brown (beige) while the back face is some kind of greyish-bluish. You can however change this at SketchUp (Window on PC) > Styles > In model (the small, house icon) > Face settings (the second icon on the top). See attached image (I'm on a PC so the dialog might be a bit different but you can get the idea) where I deleted one of the faces so as to reveal the back faces symbolising thickness oriented inside.
FrontFace-BackFace.png
The best is to get used to correct orientation at the very beginning of both your learning curve as well as the beginning of any (later to be complex) model. You can always right click on a back face (or a whole selection of back faces) and select "Reverse face" from the context menu. Also, you can right click on a properly oriented front face and select "Orient faces" although with more complex models (especially when there are excess faces inside a "solid") this may not work properly.
There are also plugins that automate correct face orientation with more or less success.
