Money
Money is a monetary token used for trade. 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 was often established in place of it 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.
Money and /tg/
Being a tabletop hobbyist, money plays a huge role in the life of every neck beard. If you don't have money you can't build your army, without your army you cannot participate in tabletop match. The require products for this hobby comes with miniatures, paint and codex. They are all damn expensive, especially when they are all measured in euro (which is currently the most expensive currency of all).
Money in games
Money is a common feature in many games. It's an idea you would naturally be familiar with, is a natural part of any agricultural society not designed by the dirtiest of hippies and is for providing incentive to players in story creation. On top of that is easy to express with a pen and paper. Your Paladin has acquired 50 Golden-Wombats so add fifty Golden-Wombats to their inventory, you subtract 30 Golden-Wombats from your inventory when you buy a new shield from a blacksmith and so forth.
In settings where the system of currency has collapsed or is non-existent, like a post-apocalyptic world,, money is either eliminated and people stick to bartering or something else more useful is used in its place. The Metro 2033 universe, for example, has people trading in pre-war ammunition after Moscow's economy was obliterated in the setting's nuclear war.
Money in Fantasy
The default type of money in you usual fantasy world are coins following a simplified version of the Roman System with (in descending order of value) Gold, Silver and Copper pieces. Such systems are usually made by people who are either lazy or don't hold the nature of local currency to be of much concern of the worlds of the story that they're writing. Those that go more in depth or want to differentiate different cultures and civilizations will add different types of money. Those who want to simplify things even further simply stick with gold. Gold's density is usually left unaddressed; a typical "fantasy coin" (1" diameter, 1/8" thick -- gold is too soft to be made into thin coins) of pure gold would weigh a little over an ounce (31 grams to be precise), so a sack of 100 coins (enough to buy a decent sword or an adventure's worth of supplies) would weigh nearly seven pounds, and never mind lugging around 1000 gold coins (68 pounds) to buy a suit of armor.
Another challenge of gold specifically is that one of the main goals of alchemists was working out a way to transmute common metals into gold. In a few fantasy worlds there are various types of magic that can do this. Such magics would inevitably lead to inflation. This might explain why gold coins are used as a day to day currency in many fantasy worlds when in real life it was unusual for people who were not merchants, nobles or scribes to deal in gold rather than in silver or copper. One idea that /tg/ had was the use of solidified magic as a form of currency.
For some reason dragons also like to sit in piles of gold and gems, which makes them targets for adventurers trying to seek wealth.
Wouldn't it be great if coins were made of harder, less dense metals so they could be thinner and lighter? Or heck, we could just dispense with the metal and write down how much we're worth or spending on pieces of something really lightweight, like paper. Nah, this is crazy talk!
Money in Sci-fi
In futuristic settings, money is usually just a single unit (typically called "credits") and largely operate on a digital level instead of people having to give a form of bank note or coin. In most sci-fi settings that include aliens with their own currency system, the proverbial credit is usually the standardized form of money people pay with that has a direct equivalent to their home system. Basically similar to what the European Union did with the Euro, but on a larger scale.
In WH40K; currency is not completely detailed upon. The Imperium officially runs on a feudal system with no unified currency system, where each world must pay tithes of various resources such as food, minerals, hardware, manpower.....etc, instead of paying in coin. Although several publications say that each Imperial world has their own local currency system, not to mention that guardsmen also get paid with actual money (although unspecified). Most people typically just accept that Imperial currency is either called "credits" or "thrones" (throne gelt, specifically, and yes that does indeed mean they are Golden Thrones), since they're the most prevalent.
Additionally, every human of the Imperium is indebted to the Emperor at birth, as the only reason they're alive is because of the Emperor's continued sacrifice upon the Golden Throne. Thus, every citizen of the Imperium is considered a currency of the Emperor, and can be spent by the billions to preserve it. That said, wasting something that belongs to the Emperor himself is heresy, so while a commander or an Inquisitor may have the authority to spend that much if needed, they better have a good reason for it (after all, those people could have been used for something else more productive).
For Orks, they use teef, which can be obtained by krumping a git and yanking his chompers out. To orks; bigga teef means bigga value, so Orks who wants the killiest gubbins from the meks typically has to pick on someone his own size, or someone larger. Orks also run on a bartering system, where one can trade something of value; like salvage or resources to get what they want. Because teeth rots overtime, it cannot be hoarded for long, and has to be spent constantly to get the most out of it. And because no Ork can get rich by just sitting on his ass, this helps keep an Ork band eternally thriving as the constant fighting helps stimulate the Orks' growth.