Nothing extra to pay, no reservations needed unless you want to travel on the Glacier Express or some Panorama cars on tourist routes, and it's valid on most trains in Switzerland. It’s not valid on some mountain railways. It’s not valid on trams or city transport.
Easiest way is to look at this map. It’s valid on the red lines (and red dashed lines, which are tunnels!) but not on yellow lines (buses). It’s not valid for free travel on red dotted lines, but you get a discount on the full fare.
https://www.seat61.com/reference/swiss-train-map.pdf