Little bits of
metal worth
money.
Typically, these are round and flat (discs), but different countries do this differently.
Originally coins were simply stamped pieces of metal that didn't need to be weighed because the stamp certified what weight and purity they were. Later metal money was replaced with paper that signified the original metal. These days, money has no intrinsic value except what is assigned to it by the market, and coins simply represent (usually small) portions of that kind of money. To "coin" originally means to stamp metal as coins are stamped to put the cute stuff on each side that makes it a "coin" instead of a disc of metal.