Depending on your booking, your rental includes 1,000 km, 1,500 km, or 2,000 km per month. If you go over this package, you'll be charged for the extra kilometers at a rate of €0.25 incl. tax per additional kilometer driven.
Example, for the 2,000 km package:
If you drive 2,500 km during your first month of rental, you'll be charged 500 × 0.25, for a total of €125.
The overage is calculated at each monthly due date, and you can track your mileage in the app, under the "Trip details" tab.
Any kilometers you don't use in a given month roll over to the following months, as explained here.