The Farm is the basic agricultural improvement in Civilization VI and is available from the start of the game (no tech required). Initially, it can only be constructed on flatland Grassland, Plains or Floodplains tiles, but researching Civil Engineering enables Farms to be built on Grassland and Plains Hills.
Farms are one of the two basic Tile Improvements (along with the Mine) which can be built on land without a Resource. They are the main booster of Food, and along with the Camp and the Pasture provide additional Housing since the very start of the game. This makes them the main city growth booster throughout the game. Farms can be built on non-desert and non-tundra flat lands, which are the most available tiles in Civilization VI. However, unlike other improvements, Farms' yield doesn't improve automatically with technology - rather, they gain Adjacency bonuses.
To utilize the adjacency bonus of Feudalism starting with Medieval Era, a farm needs to be adjacent to two more farms. The best formation for this purpose is a triangle of tiles (which could be expanded to a diamond-shape of four tiles). This way every farm in the formation gets adjacency bonus. It is worth noting that the adjacent farms do not have to be from the same city or even the same civilization for farm adjacency bonus to trigger.
Later in the game cities require a large amount of Food to maintain growth. The only way to achieve large Food production is through the adjacency bonus of Farms. So, plan ahead where such farming triangles and diamonds can be placed, because you will be using it for most of the game (the same layout will also serve to trigger the adjacency bonus of the later Replaceable Parts from the Modern Era). Usually, there would be multiple areas available per city at first. But soon, various Districts and Wonders take over the available spots. Use Map Pins to plan ahead where those Districts and Wonders will be and identify where your primary Food tiles will be through elimination. Civil Engineering from the Industrial Era enables Farms to be built on Hills. That will give more flexibility to farming triangles and diamonds. It is ok to remove and rebuild tile improvements over time. However, placement of Districts and Wonders is permanent. If you haven't planned ahead, you may find no possible ways to have significant Food production for an old city, which is also likely an important production center.
Civilopedia entry Edit
Once people figured out that it was a lot easier to stay in one place and grow crops, rather than wander about gathering whatever happened to be growing, "civilization" broke out. Around 12,000 years ago, this was the "Neolithic Revolution." Farming meant that permanent settlements could be self-sustaining, and that the more food that could be harvested, the bigger settlements could grow. Ever since, advances in technology have been making agriculture more efficient: irrigation, machinery, fertilizers, pesticides, GMOs and so forth. Yields skyrocketed.
The Civilopedia entry states that "Constructing farms adjacent to other Farms provides bonuses to Housing." This is incorrect.
You can tell by the looks of a Farm whether it is currently being worked or not. Worked Farms appear green and lush; otherwise, they appear washed-out, and dull.