Before the invention of money (long long ago, in a galaxy not so far away), people used barter in order to get the things they didn't have enough of. This became quite cumbersome, so they agreed on a representative unit of wealth . Rather than having to carry around anvils, or fish, or wool whenever you had to buy something, they instead carried around this unit of currency - in many cases, it was gold (today, it might be dollar s).

At some point in the history of banking, this unit of currency was no longer used, but was instead replaced by the writing of paper checks from one bank account to another. However the gold remained in the bank, even though it was no longer used - this was an attempt to give some legitimacy to the new paper notes being used - the gold standard.

The problem with this idea is that it is similar to the creation of a "dollar standard" - creating a new currency to use, while holding a reserve of dollars in the bank to give the new currency some legitimacy. The problem is that the commodity held in reserve was merely a unit of exchange and derives its value mainly from its previous use as currency. The original backing of the currency is lost.

The Sumerians, as part of their development of a standard of weights and measures, placed the royal stamp on each piece of gold to guarantee that it was the same amount as every other similarly stamped gold piece. They simply agreed that this was worth a bushel of wheat - the value was never in the gold. For each amount of gold issued by the king, a certain amount of wheat is kept in reserve in order to ensure that gold has some value. This ensures that the value of the gold with respect to wheat did not change - no inflation. When the gold is returned to the king, it is redeemed with the wheat that it represented. This, in effect, is a "wheat standard".

(Thanks to Gritchka for help.)