Frankfurt to Salzburg

Your options: https://www.bahn.com/en/view/booking-information/index.shtml

Are you arriving by air the day you'll be travelling to Salzburg, or will you be in Frankfurt for a few days before that trip? If you'll be travelling direct from the airport, you'll likely be using the Fernbahnhof (for long distance trains) and there are ticket offices and ticket machines there.

It would help to have more information in order to provide you with more specific suggestions.

Will be arriving by air in Frankfurt then going directly to Salzburg by train the same day.

Try to get a Rail&Fly ticket from your airline. If that is Lufthansa: they charge 34 Euros and they allow adding Rail&Fly to an existing booking. Not all airlines allow this, though.
If you cannot get a Rail&Fly ticket, the fare will be considerably more expensive.

Good advice about Rail & Fly. If you aren't flying Lufthansa, downtown the Deutschebahn app onto your phone, purchase your ticket via the app, and then the ticket will be in the app. If that's too technical, purchase the ticket via the Bahn.com website and print it out to have with you.

Another vote in favor of the Deutsche Bahn App!

purchase the ticket via the Bahn.com website and print it out to have with you.

... and then download it into the App using the booking reference #. So, if your ticket should get lost it's always recoverable easily.

"Will be arriving by air in Frankfurt then going directly to Salzburg by train the same day."

Options:

DB saver tickets = pre-set train schedule, no flexibility for you if there's a flight delay and you miss the train.
DB full-fare tickets = very expensive.

Rail & Fly tickets allow you to travel without a set schedule - IF you can arrange them.

The unsolved "elephant-in-the-room" problem is that after an overnight international flight across several time zones, you'll be climbing immediately on a train, unwashed and probably unrested, for another 5.5 - 6.0 hour journey!

What you might not know is that you can perhaps buy a cheaper DB saver fare to Salzburg with an overnight stopover in Frankfurt and finish your journey to Salzburg the next morning. Schedule a local or regional train into Frankfurt proper for the 10-minute train ride on Day 1, then the rest by high-speed train the next morning.

Just an example on May 10: Fare is €39 pp at the current time:

May 10 FRA - Frankfurt Hbf 9:47 - 9:58
(Entere a 19-hr. Stopover in Frankfurt so you can sightsee, relax, sleep, get up early - which is probably unavoidable anyway.)

May 11: Frankfurt Hbf - Salzburg 5:17 - 11:42
(Second "stopover" in Stuttgart with 00:00 stopover time, which routes you for this journey and this fare)

So besides the stopover trick, I entered "Hallstatt" as your final destination for this trip and this ticket price, which routes you through Salzburg on the way (and you just get off in Salzburg.)

Now what about that 9:47 train from FRA to Frankfurt, you ask? No problem with this ticket - you can ride ANY regional or local train into town with this saver fare ticket! DB doesn't care about those - really.

Interesting, Hidden City Ticketing. And Unlike the airlines, Deutsche Bahn doesn't care and really does not have any way to track that you "missed" your connection.

Interesting! Thanks for all the great information. Unfortunately can’t overnight in Frankfurt as have to be in Bad Ischl the next AM. Will save that for the next time😀

So at this point, you might as well just go to the Fernbahnhof after you land, get through immigration and get your luggage and buy a ticket. Costs 128 EUR for 2nd class, and pay the extra 4.50 EUR for a seat reservation because trains look like they could be fairly crowded. There are trains about every half hour and it will take about 6 hours to get to Salzburg.