You'll need a serial terminal application such as minicom, cutecom, gkermit, gtkterm etc. to interact with it. Connect to whatever the usb serial device appears as - probably /dev/ttyACM0 or /dev/ttyUSB0. Many of the USB serial devices auto-sense the connection parameters, but you might need to enter them manually. Mine should arrive in a few hours so I might be able to offer more detail after that if you're still having trouble.
Edit: connection details in the postmarketOS wiki
Edit: connection details in the postmarketOS wiki