As you try to purchase upgrades you may find yourself limited in terms of resources. The trade menu allows you to trade resources with other players.
Trade Offers are deals that have been created by other players. You can filter through the available offers to find trades offering the resources you want.
Once you accept the trade, the other player gets the resources the exchanged (minus a trade tarrif which goes to the rulers of both kingdoms involved in the trade).
Make your own trade offers
You can also set up your own trade offers through the Make a Trade section.
Again bear in mind that some taxes will be added by your rulers, so make sure that the deal you offer is appealing!