Solid mesh and Hollow mesh?

Hi, I’m trying UE5 to visualize my model made in Blender. Now, theres a vizualization mode called distance field which shows nearest surface of a mesh. However, for some reasons, some meshes are shown as an empty mesh. Its true that when I was modeling in blender, I didnt extrude some meshes, but created by connecting faces from vertices. Would that make this mesh as a hollow mesh?
Im trying to understand the concept of a hollow mesh and a solid mesh. Thanks


empty (?), hollow, solid…

IDK is this are proper names… i know manifold and non manifold edges… also maybe inverted normals on some faces… so…

You may have to elaborate this to get a more suited answer.