Writing an option means selling a certificate that guarantees the holder can buy a traded item for a guaranteed price (strike price). The person who writes the option is betting that the price of the underlying stock will go down (shorting a stock, AKA a short position). If the person writing the option is correct, then she makes money off the sale of the option, but does not have to worry about honoring the option, since it is out of the money and has no intrinsic value.
If the person writing the option is wrong, and the price of the underlying stock goes up, then she must buy the item at the higher spot price specifically to sell it at the low strike price ("short cover"). In rare cases, a person who makes this sort of error will actually drive the spot price much higher than it would have gone ordinarily.