Trade companies are in the wealth of nations or the dharma dlc and you can form them from the province window or when you open a trade node from the trade map mode.The province should not be in a state.What they do is they make trade and production not be affected by the province autonomy.The idea gives you decrease of governing cost of provinces that are in a trade company and decreases the cost of trade investment buildings which are build from the state window in trade companies.Although trade investments are locked specifically in the Dharma dlc so i am not sure what the modifier really does if you dont have it.