Lots, or contract size, are the standard amount that each contract is worth.
For example, The XAUUSD pair tells the trader how many US Dollar (the quote currency) are needed to purchase one Gold Ounce.
The contract size for XAUUSD is 100, that means, 1 lot XAUUSD's price movement would be equal to 100 Gold ounce.
aka. for 1 lot CFD,
CFD Value Change = Price Change * Contract Size