Money

Money is a monetary token used for trade. Because bartering using resources was typically not always a reliable form of commerce (as value of goods typically varies from place to place; one area may favor the goods you're trying to trade with, while another area might consider it worthless.), a centralized form of payment is established so that people would have an easier time in gauging the value of a product and trying to pay for it (after all its much easier to just carry some gold coins around than a cartload of goods). Lots of things have been used as money from fancy shells to grain to plugs of tobacco to valuable lumps of metal to notes backed by the authority of government. MOST IMPORTANTLY, money allows YOU to buy cool toys so that you can become part of /tg/ while keeping another money-whoring company alive. Show them your love of currency by paying for your next box of Space Marines with a Yap Stone.

That said, money only works if the place you're trying to spend it at has use for it (after all, all the money in the world is useless if you can't get anything with it). A town with a well-connected trading post will welcome your coins with open palms, while an isolated village in the middle of nowhere would find those hunks of metal to be worthless and would prefer you give them something actually useful in exchange instead.