If you have a device connected to a serial port of your computer, you can communicate with it by typing M-x serial-term. This command asks for a serial port name and speed, and switches to a new Term mode buffer. Emacs communicates with the serial device through this buffer just like it does with a terminal in ordinary Term mode.
The speed of the serial port is measured in bits per second. The most common speed is 9600 bits per second. You can change the speed interactively by clicking on the mode line.
A serial port can be configured even more by clicking on ‘8N1’ in the mode line. By default, a serial port is configured as ‘8N1’, which means that each byte consists of 8 data bits, No parity check bit, and 1 stopbit.
If the speed or the configuration is wrong, you cannot communicate with your device and will probably only see garbage output in the window.