Manna is a form of food that appears randomly. It is the main source of energy in Grobots, typically accounting for more than half of all income.
The hardware for gathering manna has a high return for its cost, so it allows sides to grow much faster than solar cells. The supply is limited by available territory, though.
Sides that live mostly on manna are called gatherers (or theotrophs, by analogy to biological terms).
The rate at which manna appears can be adjusted in the Rules dialog.