Very sorry for the late reply on this question. Yes that is the correct meaning of “overlapping parts” and the problem with overlapping parts is sometimes it makes the model glitchy and ugly, it also creates more un-needed polygons which will make changing the model even harder and a headache and probably make the game lag I don’t know.
I will now show an example of this problem. Here is a simple cube. See a problem with it?
The problem because it looks like a cube with normal 12 tris but nope, it has 52. That is because of overlapping faces. If I were to try and change this cube by editing it than it would be hard to do. It’s just better to get rid of overlapping faces.