CommCat connects to a VHF/UHF Packet repeater through a radio and TNC (Terminal Node Controller). The TNC converts audio tones coming from the radio into data, and data into audio tones to be transmitted and PTT control for the transmitter. The TNC is connected to a serial port on your computer.
See the CommCat Packet settings topic for more information on configuring CommCat to communicate with the TNC and your packet node.