Sunshine, the difference between the Saver Day Pass and a Swiss Pass is:
1) A Saver Day Pass is for one day only.
2) It has variable pricing, you MUST buy it in advance and it is cheaper the earlier you buy. If you leave it too late it could be sold out for that day.
3) It gives a discount if you already hold a Half Price Card.
Details here: https://www.sbb.ch/en/travelcards-and-tickets/tickets-for-switzerland/1-day-travelpass/saver-day-pass.html
The coverage is the same as a Swiss Pass (or a "GA" the annual pass residents can buy), that is all transport, rail+bus+boat+city transport, excluding some mountain railways which are purely for tourists.
@Jules M, yes many Swiss towns and cities include local transport if you are staying in a hotel, but that is only good for that town or city, not if you want to do a day trip to somewhere else (Zürich, Engadin, . . . )