The game does a terrible job of explaining how all of these bonuses work. The Potkop basically is an upgraded Farm, but it has some advantages that may not seem immediately obvious. Because it generates both food and gold, the game considers it to be both a food and a gold building, and it picks up any bonuses that you have for either of those things. It's a very situational improvement, though--you only want to place it next to resources, and because it can't be built over, it makes a lot of sense in towns but maybe less sense in cities (much like warehouses).
An example from my last game: This is the Stepwell and not the Potkop, but the same rules apply. The Stepwell generates 2 food per adjacent farm plus 2 gold (just a flat amount), so it's really a "town only" sort of improvement--you don't want to place it in a settlement where you think you are going to eventually urbanize your farms. So, I place one on a farm that is between two other farms. This takes the tile from yielding 6 food, 1 production, and 1 gold to yielding 9 food, 1 production, 2 gold, and 3 culture. The culture comes from a policy I have slotted that gives +3 culture to any gold building; the farm wasn't considered a gold building, but the Stepwell is. It's also a food building/farm, so it will get all the bonuses applied to those, too (including warehouse bonuses).
These improvements are highly situational, but they can be worth building in the right location. For the Potkop, a farm tile next to one resource is probably kind of marginal--it's an increase in output, but maybe not enough to be worth building. Maybe if you have a boost to gold buildings slotted, but otherwise probably not. A farm tile next to two or more resources, though, is a place where it definitely makes sense to build a Potkop.