He understood you.
It's whatever you want it to be. It's completely irrelevant to the mechanics of the game so you can decide for yourself if it's some small mech, or some droids, or a dude in an armor suit, or 10 guys with pistols in police uniforms.
As far as the combat mechanics go it doesn't matter. Each unit has x armor, fires so many shots, with so much penetration, using so much supply.
So use your imagination.
Maybe they're trained space slugs that shoot armor penetrating poison?