If you're not buying Advanced Purchase tickets, eg you're getting an Off-Peak day return or something like that, you can just buy it at the station, and a nice member of ticket office staff can explain these things for you!

You can read about the off-peak break of journey rules on the National Rail Enquiries website here

When booking your tickets, on any sensible website, it should show you the different ticket types available for your journey. If you're happy to save money by going through Salisbury both ways, you'll want the cheaper one with the restriction "Route Salisbury", rather than the more expensive "Route Any Permitted"