A pass with a validity of 3, 4, 5 or 8 days within 1 month means that you're free to choose any 3, 4, 5 or 8 travel days within the overall validity period of 1 month. These 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 8 travel days for this trip, because all of these cities can be reached in a day. So instead, you buy a 3 days within 1 month pass, which is cheaper and suits your trip better.