From Outscape Wiki
Home guard refers to a planets inherent defensive strength expressed as a small percentage (5%) of its population that are fit and healthy (and trusted!) enough to be allowed arms in the event of a Planetary Assault.
These "bonus troops" add to any army the planet has from Military Bases and have to be overcome by invaders. Think of them as the "National Guard" or "Territorial Army" or "Militia" of each planet.