Hello Thiago
With a Saver Day Pass, you can travel throughout Switzerland for an unlimited number of days. You can also use it to travel the routes you specify as long as they are within Switzerland. Please note, however, that the Saver Day Pass is only valid for 2nd class.
In terms of reservations, it depends on the type of train you will be taking. If you are taking a regular train, no reservation is required. However, if you wish to take a panoramic train such as the Bernina Express, a reservation is required. Please note that there is an additional charge for the Bernina Express, even if you have a Saver Day Pass.
Have a wonderful day.
Kind regards,
GiuseppeL