InterRail passes are valid for different periods of time and with a different number of travel days*. 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 InterRail pass is valid.
If you’re planning to travel by train on most days of your trip, a pass that’s valid e.g. 15 days 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 take unlimited trains on a select number of days, pick a pass with which you can travel e.g. 5 days within 10 days. You can choose on the go when you use each of your travel days during the overall period that your pass is valid.
* The number of travel days available varies per pass.