Lot (contract size)
A lot is the standard quantity an instrument trades in. Shares traditionally traded in round lots of 100; a futures contract has a fixed size set by the exchange, such as 100 troy ounces for one CME gold contract.
Lots decide the smallest step in which you can change your position. A standard forex lot is 100,000 units of the base currency, with mini (10,000) and micro (1,000) lots for smaller accounts. Futures exchanges list micro contracts, one tenth of the standard size, for the same reason. Crypto exchanges set a minimum quantity and a step instead: on Binance futures, AAPLUSDT trades in steps of 0.01.
Your position size, worked out from the money you are willing to lose, must be rounded to the instrument’s lot. When one lot is too big for your account, the honest answer is a smaller contract or no trade.
Learn more: spot vs futures.