A pass with 10 days within 2 months means that you get 10 travel days, which can be used for train travel within a 2 month period. Travel days are flexible, so you can decide on the go when you use them. Choose the pass with the number of travel days that best fits your travel plans.
For instance: You are planning to travel in Italy and visit Rome, Florence and Venice. You don't need 10 travel days for this trip, because all of these cities can be reached in a day. So instead, you buy a 3 days within 2 months pass, which is cheaper and suits your trip better.