Eurail passes are valid for different time and travel day combinations. A travel day lasts 24 hours from midnight to midnight. On each travel day you have access to the train networks of the countries where your pass is valid.
If you’re planning to travel by train on most days of your trip, a pass that’s valid e.g. 2 months continuous will give you the freedom you want. With this type of pass, each day can be used as a travel day, but of course you’re not required to travel every single day.
If you’d like to travel by train on a select number of days, pick a pass with which you can travel e.g. 10 days within 2 months. You can choose on the go when you use each of your travel days during the overall period that your pass is valid.