A futures contract, simply

A futures contract is a standardized commitment to buy or sell an asset at a future date, at a price fixed today. On stock indices like the S&P 500, nobody delivers a basket of shares at expiration: the contract is settled in cash. And for an intraday trader, expiration hardly matters, since they open and close their position within the day. What matters is that the futures price tracks the index very closely, and that it trades on a centralized, liquid market, almost twenty-four hours a day.

The great practical advantage: buying and selling are perfectly symmetrical. You can sell a contract you do not own (being “short”) as easily as you buy one (being “long”). You make money if price falls when you are short, and if it rises when you are long. No borrowing of shares, no special constraint.