Default-meshes.igs is an igs file that contains the mesh information of a scene. It is referenced from a main igs file and cannot be rendered by itself, you must open the main file (either default.igs or a set file-name) to render.
This file is created separately and referenced so that a exporter can export either just mesh data, or all other data without having to rebuild the meshes (which is usually the longest part of an export). This feature is known as "Quick Export".