Leverage is the ability to control a large contract value with a relatively small amount of capital.
Unlike in equities or fixed-income markets, when you trade futures the capital in your trading account is utilized to cover the margin (or performance bond) of the product you are trading.
This margin is typically 3-12% of a contract's notional value.
The notional value of the contract is the cash value of the market exposure you will have when you trade that product.
The difference between the notional value and the margin is the leverage of the contract.
For example, the micro e-mini S&P 500 contract has a current notional value of $23,675 ($5 * the index value), but an initial margin of $1,298 (maintenance margin of $1,180 * 10%).
This means that for $1,298 you can control $23,675 of the S&P 500 futures market, which provides greater capital efficiency.