With the Swiss Travel Pass (not the Swiss Pass, that is something else) you have unlimted travel on all public transit. So you just take trains, trams, busses, and even some cable cars at your own discretion. it is really an “all you can eat” product.
And you do not need to secure a reservation. Just find the train you want to take and get on.
Mountain excursions and cable caras are not part of the public transit network, so you pay extra for those. So if you for example want to go to Oeschinensee you first need to get to Kandersteg. The trains are covered by your pass, and so is the bus to the cable care station. But the cable car itself you will need to pay for. That you can when you are there. The Swiss Travel Pass does often give you a reduction though, so always tell the ticket clerk that you have one.