The Farm is a resouce node that will produce a certain amount of food per hour, based on the amount of citizens stationed in that node.
These nodes are of minor importance until late game, as the bulk of your resource income should be from attacking camps, and to a lesser degree through income from villages.
Later in the game these nodes can be useful to supplement your resource income, and to act as an alternative to gold. Once the market price for resources levels out and your production technology is maxed out, it can often be worth more gold to produce a resource per hour and sell it, than it is to produce a gold per hour.
Once again these should NOT be your primary source of resource income. That should be from camps and to a lesser extent villages. Otherwise you will grow at a snails pace.