When you rent a SnappCar, you get a certain amount of kilometers per rental day of your rental period you are allowed to drive without an extra charge. The amount of free kilometers is set by the owner, as is the price a renter pays per kilometer when the renter drives more than the free amount. The total amount of free kilometers for a rental is calculated by multiplying the number of days the rental period is long by the number of free kilometers per day.
It doesn't matter how many kilometers you drive each day of the booking. As long as the total amount of kilometers driven are in the included range, it will not cost extra.
This cost is separate from the cost for fuel. Always communicate with the owner about how to pay for the fuel of the booking, since that is not part of the price you pay through SnappCar.