You get an extra trade (but subject to Despotism's maximum of 2) in each of the following cases:
- there is a road and you have researched The Wheel
- there is a river and you have researched Map Making.
Plains are the equivalent of the Civ series "Grassland with resource". The C-evo "prairie" is the equivalent of Civ plains. For more food in C-evo, once you have emerged from Despotism, you need Grassland.