Simply put, gold is the most important big deal in history.
Gold (symbol Au, atomic number 79) is a yellow, shiny, soft, dense, rare, corrosion-resistant, and very valuable metal — the last property means that it turns up heavily in tropes, both figuratively (signifying purity and value) and literally.
You know gold is heavy, but it's almost certainly heavier than you think. It's denser than lead, and on the level of uranium. One of the things many people who handle gold coins for the first time remark on is how dense the coin feels when held in one's hand. The History Channel interviewed a guy who had a part-time job ferrying gold bars to and from the vaults for an internal audit of Fort Knox back in the 1970s. He mentioned how a brick of gold is heavy (weighs about 33 pounds), is dirty (the dust isn't cleared out of sealed vaults), and is dangerous (drop a gold brick on your foot and you'll break every bone.) For the same amount of space, gold is about ten times as dense as brick. Lifting a single bar is something you do with both hands, carefully. Lifting a crate of gold is done with heavy machinery. Fiction rarely gets this right.
Gold is valuable in part because it is so rare, much rarer than it is commonly depicted in fiction. A total of about 160,000 metric tons of gold have been mined throughout all of human history, or 8500 cubic meters as a solid mass of pure gold. Assuming Scrooge McDuck's money bin is a cube six stories tall, filling it with gold would require 2/3 of the world's gold supply. A major factor in its rarity lies in the fact that it doesn't naturally occur on Earth — scientists believe that it typically appears as a byproduct of supernovas and that it was deposited on Earth through meteor strikes. Given the low chance of any hunk of rock entering Earth's atmosphere without vaporizing instantly, this means that Earth only has a finite amount of gold, and once that runs out, the chances of replenishing it are exceedingly slim.
For most of human history, gold's unique properties affected how it was used. On the physical side, it's too soft and dense to be made into practical tools; but its ductility, looks and low reactivity allows people to craft all kinds of artwork out of it. Because gold is rarer to obtain than other elements through pre-industrial revolution processes, and it doesn't rot, rust or tarnish either, it's always been a popular way to store wealth.
Thanks to the special place gold has in society, it is very significant in human cultures, and a lot of symbolism has been applied to it: in alchemy, turning lead into gold is one of the properties of the Philosopher's Stone, together with making men immortal, much like gold itself is an immortal, incorruptible metal. This also colors gold's portrayal in fiction.
These days, however, gold has a wider range of uses. One of the most popular uses is in electronics, as it is one of the best conductors of electricity. Gold is slightly less conductive than copper and silver
but is highly resistant to corrosion compared to them. Those silicon chips in your computer? They're connected to the pins on their casings by very fine, very pure gold wires. The aforementioned corrosion resistance means that this is the only way wires could be that fine; copper or silver wires would just turn into non-conductive oxide. Recycle a tonne of old computers and you can recover a whopping 250 grams of gold. Gold's resistance to corrosion also makes it popular among audiophiles, with a number of luxury imprints from the late '80s to the early 2010s releasing Compact Discs with reflective layers made of 24 karat gold instead of aluminum to avoid disc rot, a rare but highly damaging event where the reflective layer in an optical disc oxidizes away with time (usually the result of a manufacturing defect, poor storage conditions, or a particularly deep scratch), rending the disc increasingly unreadable. Gold-plated cables are also common audiophile equipment, to the point of them becoming popular bywords for the scene.
For more raw facts about gold see Wikipedia's article on the stuff.
For tropes about gold, both literal and figurative, see The Gilded Index.
Not to be confused with the British TV channel or the Spandau Ballet song.
