What does Editor Only Static Mesh Component mean in the Building Prop BP?

Hi.

Can someone explain or point me to some documentation that explains what Editor Only Static Mesh Component means?

It means that it only will exist within the Unreal editor. A typical use case would be having an invisible wall, where the static mesh component uses an invisible material, while the editor static mesh component uses a visible material, so designers can see where the invisible wall is while working, but the players will not.