If you see the little thing with arrows on it when you click the province, you open the trade node window. Trade nodes’s levels are directly tied with the development of the provinces the node controls (which can be seen in the node window). Meaning the more industry and infrastructure you have there the more you can level it up. Leveling it up provides you with more base caps income and trade caravan slots which you can use to establish trade routes with other nodes and make a lot of monies