The ESP302-RS232 is an adapter for the 15-pin D-SUB-HD COMM. connector of the ESP302, that converts the signals to a 9-pin D-SUB RS232 serial port connection.

See section 6.3.1 in the "ESP302 -  Start-Up Manual", which is available for download in the "Resources & Download" section below, for more information about the serial communication port.