In the old days, you couldn’t afford a computer, so you talked to your TNC (it cost enough as it were) with a “dumb terminal”. A kind of “glass teletype” machine. The famous one is the DEC VT-100, which was used for ages to talk to mainframe computers…. anyway…
When computers did come around, you needed some software running to talk to the TNC. A Terminal Emulator for Windows is Hyperterminal.
In Linux, you can use Screen or minicom.
Here is a link to do with running minicom with a TNC with a RaspberryPi.