Look at the way the textures are applied to a fighter, then at the way they're applied to a capital ship.
Then open up the original Aeolus and the new high-poly Aeolus.
UV Mapping (in this context) refers to using one, large, high-resolution texture over the entire ship instead of multiple smaller textures and tiling them. The original Freespace engine got better performance with smaller textures; the new one gets better performance with fewer textures.
Technically, UV mapping is the process of applying texture coordinates to polygons. Whereas each face has X, Y, and Z coordinates (its location in 3D space) it also has U, V, and W coordinates (its location on the texture map.) Every textured model has UV coordinates but here we're talking about the painstaking process of combining all the tile textures onto one huge texture and reassigning UVs for each face in the model.