The trade potential is an abstract measurement which defines the possibility for a holding to earn additional gold which goes directly to the treasury. For annexed provinces the potential is calculated on the basis of population number of two adjacent provinces and the less populated holding determines the potential. The more villagers there are, the greater the potential.
100 population grants 2 gold (for fast realm)
e.g. If the province has 50 000 population (without the homeless population) -> 50 000/100*2 = 1000 trade potential of the province.
But, trade potential can't be utilized without the construction of imperial routes between provinces. Every level of an imperial route grants 10% utilization of the potential (of the less populated holding).
e.g. The imperial route between the provinces has 2 levels and 20% of the trade potential of the province with less population is utilized-> 200 gold income from the province with 50 000 population
Colonies follow the same principle, only you need to construct a trade route in order to use the potential.
Trading posts don't have population and the trade potential is determined by the type of special resource they lay on.
Special resources table in
http://www62.imperiaonline.org/imperia/game_v6/game/help/?section=global_map&lang=en&speed=2#globalmap_22 .
The current income from imperial and trade routes can be checked in Imperial routes menu -> The whole empire tab.
