The power supply to the module (usually 12V) and the serial ports use screw connectors. The connectors have printed labels in the PCB. The labels for the power supply are 12V (positive) and GND (negative). The 2 serial input ports have labels A1 and B1 (port 1) and A2 and B2 (port 2). If your Nmea data is available though a differential port you connect the A (or +) terminal to the module A1 (or A2) and the B (or -) terminal to the module B1 (or B2). If your Nmea data is available through a single ended port (such as RS-232) you connect the Output terminal to the module A1 (or A2) and the Ground (or common) terminal to the module B1 (or B2). The output serial port uses a 3-way screw connector with labels A3 B3 and GND. If the receiving device has an opto-isolated port, you connect A3 to the A (or +) terminal of the receiving device and B3 to the B (or -) terminal of the receiving device. If the input port of the receiving device is not opto-isolated you should connect A3 to the input terminal of the receiving device and GND to the common (or ground) terminal of the receiving device. In that case you should not use B3.
Important: the module is now wifi bidirectional (version 4.0). Please check the forum on this site for more information.
The pictures in this set refer to version 3.0. Present version is 4.1 and wifi bidirectional. The last picture is a "test" screenshot of SeaWifi (obtained here) showing 19,385 NMEA sentences being received at a rate of 16 per second and with zero checksum errors.