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!
Trades are subject to taxes. Players not in a kingdom pay a 10% tax I.e. they get 90% of the resources that were originally offered by the offering player. The taxes paid by players in Kingdoms depends on the type of kingdom, and the relationship between the two kingdoms making the trade. The numbers you see when accepting a trade offer already take the taxes into account, but you should bear tarrifs in mind when making trade offers as the players seeing your offers will likely not get the whole amount you are offering.