Gold as money
The commodity that anchored currencies for centuries, and no longer does.
Gold became money for unglamorous reasons. It does not corrode, so it stores. It is soft enough to strike into coins but scarce enough that striking more is difficult. It is uniform, so one ounce is interchangeable with any other. Those properties made it a workable store of value long before anyone theorised about why.
The gold standard
Under a full gold standard a country's currency was legally convertible into a fixed quantity of gold, and the money supply was constrained by the gold held in reserve. This imposed discipline, a government could not simply issue more currency, but it also removed the ability to respond to a downturn by easing monetary conditions. That rigidity is a large part of why the system broke.
The Bretton Woods arrangement after 1944 was a partial version: other currencies were pegged to the US dollar, and the dollar alone was convertible into gold at a fixed price. When the United States suspended that convertibility in 1971, the last formal link between circulating money and gold ended.
Where gold sits now
No circulating national currency is redeemable for gold today. Central banks still hold substantial gold reserves, and gold is still bought as a store of value and a hedge, but it functions as an asset rather than as money. You cannot spend it, and its price floats like any other traded commodity.
A caution about "gold-backed" claims
Because the idea carries historical weight, "backed by gold" is a common feature of currency investment pitches: schemes promising that some obscure or defunct currency will be revalued because of gold reserves. Treat such claims with suspicion. A currency's exchange rate is set by its central bank and by markets, not by a reserve story, and no legitimate central bank announces a revaluation through investment newsletters.