Food is one of the resources in the game. You can produce it at a colony by building farms. The amount of food your colony produces can be seen by clicking the resource summary button on your colony overview screen. Food is the only resource which is not required in construction of buildings or ships.

If you don't produce any, the private economy will farm for themselves, and purchase any remainder from the global market. Colonists will buy any food you produce or import for 25 credits per unit, up to the amount they need for consumption.
Food from the global market will cost them a lot more and might even completely drain their wealth.

25 people consume one unit of food per day.