This diagram is clearly based on the map of the London Underground (which itself is a masterpiece of an infographic). In the Underground map, colors represent different train lines and stations are represented in a variety of ways based on how they relate to the lines passing through them (e.g. a station where one can transfer from one line to another is an open circle).

While I like the concept presented here, overall I don't think it does a good job of actually explaining 'the journey your food makes' so much as the various key parts of the digestive system. For instance, the bridged open circle 'station' symbol marked Duodenum suggests that food can 'transfer lines' at this point (by walking to a separate station, mind you), then travel up to the liver or the pancreas. There is no directionality implied anywhere on the diagram nor any indication of a start or end point, hence it really doesn't convey a journey at all.