What does 4, 5, 6, 8 or 10 days within 2 months mean?

A pass with 4, 5, 6, 8, or 10 days within 2 months means that you get 4, 5, 6, 8, or 10 travel days during which you can travel by train within a 2 month period. Travel days are flexible so you can decide on the go when you use each one.