Futures are covered in full under Futures Contract, which explains how the contracts are priced, margined and settled. This page gives a short definition and explains how the plural word is used.
A futures contract is an agreement, traded on a regulated exchange, to buy or sell a fixed quantity of an asset at a price agreed today for delivery or cash settlement on a set future date. Contracts are standardized in size and expiry, and a clearing house stands between buyer and seller to guarantee performance. Traders post initial margin and gains and losses are settled daily. Futures exist on commodities such as oil and wheat, on stock indexes, interest rates and currencies, and on cryptocurrencies such as bitcoin and ether. They are used both for hedging, by producers and investors who want to lock in a price, and for speculation. In the United States, futures markets are overseen by the Commodity Futures Trading Commission.
"Futures" is the everyday plural for these contracts and for the market as a whole, as in "the futures market" or "oil futures". In crypto, the word is also used loosely to include perpetual futures, which have no expiry date and rely on funding payments instead, and which differ from the standardized dated contracts traded on traditional exchanges.