German railpass s bahn validating stamp
The super cheap tickets are train specific bookings, and subject to availability.Don't be surprised to find different prices for different trains on the same route throughout the same day.
You will also need to select a first or second class fare.
You get a cheaper price on unlimited travel for 5, 7 or 10 days in a defined time period and they are quite a good deal.
You can order them on the DB website or buy them at specific (not all) stations in Germany.
Individual fares are dependent upon whether slower or faster trains are used; the Inter City Express is the most expensive option. ‘Hbf’ is an abbreviation for Hauptbahnhof or ‘Central Station’.
I checked individual fares on both German- and English-versions of Deutsche Bahn’s website.