Faces in SketchUp have fronts and backs. The default back face color is blue. As far as 3D printing goes, the face orientation tells the printing software which side gets the material and which side is air so correct face orientation is critical.
Internal faces are inside the 3D geometry. They don’t serve any useful purpose and also create problems for 3D printing.
Keep face orientation correct and keep your models clean. It makes your work much easier.
One easy to understand definition of solid as far as SketchUp is concerned is that every edge must be shared with exactly two faces. No more, no less.
In your model where there were missing faces on the bottom of the horse, there are edges bounding only one face, not two. In the join between the N and I in Serenity, the vertical edge is shared by 4 faces. If you look closely at my version, you can see that I provide a little separation between them.