The mged tops command displays a list of all top-level objects available in a .g geometry database file. BRL-CAD .g geometry database files may contain multiple models that can be represented as a set of directed acyclic graphs. The MGED tops command lists the tops of those graphs, usually indicating the major objects/assemblies/groups of significance.
The _GLOBAL object reported by tops may generally be ignored. It contains per-file attributes such as the title and units.