When most people think of money they think of the dollars in their wallet. Money is supposed to represent something of value. For example, money could represent oil, cotton, wheat, a maufactured good, a house, a lap top, a cell phone, or anything else that is worth something and has value. However, try carrying around a barrel of oil or a bale of wheat. Money represents that barrell of oil so you don't have to carry that barrell of oil around to use for barter. It is much easier to give a person a stack of dollars rather than a barrell of oil.
However, money is more than just a representation of money. Or to put it better, real money. Real good money must ideally have low storage costs, is uniform in size and shape, be easily recognizaeable, easy to move, and last a long time.
That is why for centuries people have used gold as money. Because of it's size, composition and rarity it has low storage costs, can be molded into uniform size and shape, has a distinct color, is easy to transport, and lasts a long time.
Compare gold to paper currency. Paper currency has many of the advantages of gold as money. However, the one major weakness is that it does not hold it's value over time. Why? because people abuse it. The paper is supposed to represent something of value, but because paper is so common, it is commonly overprinted or counterfeited. When there is too much paper currency out there, it dilutes it's value. That is one of the major reasons why, paper currency representing money does not last for a long time. Paper as money has always proven to eventually collapse.