A lot is a standardised group of assets that is traded instead of a single asset.
Often, the actual value of an asset or security means that trading a single unit of it isn’t viable. In these cases, traders will use a lot: a set amount of a particular asset that is the amount you buy or sell with each transaction. It means that investors have a standardised contract and always known how much of an asset they are trading when they open a position.
A lot can refer to any asset class or financial instrument. In options trading they are often standardised across the board: an equity option, for instance, is priced so that each lot is equal to 100 shares of the underlying asset.