An Orchard produces provisions for the owning Side. They produce fruits like apples, figs, and plums. It's unknown whether they're like Farms, producing food type units (pigs in the case of farms, apples in the case of orchards) that mature over time, then depop and produce rations in a nearby city larder. Units can steal these fruits from enemy orchards.
Farm cities like Warchalking may have orchards as well, making even low level cities important to financing a side's upkeep. Interestingly, orchards (or at least fruit trees) can be built into Palaces as a means to locally produce rations. The City of Homekey had one such orchard, which also attracted applebees.