Yes, any kilometers you don't use in a given month roll over to the following months.
Example, for the 2,000 km package:
If your rental starts in January and you drive 1,500 km that first month, you'll have 2,500 km available in February (the 2,000 km February package + 500 km carried over from January).
Taking that same example, if you drive 2,500 km in January and then 1,500 km the following month (February), you'll be charged for 500 extra km at the end of the first month (0.25 × 500, for a total of €125), and you'll have 2,500 km available for March.