RS-485 is an electrical standard used to send and receive digital data over a two-wire twist pair cable, and a third common wire, typically the shield drain wire.
 
RS-485 terminals on WattNode Modbus meter

RS-485 terminals on WattNode Modbus meters are labeled A–, B+ and C, which is a very common convention. But other manufactures may simply use + and symbols or variants such as D– and D+. Sometimes the polarity of the letters may be reversed, A+, B–. The common C wire is sometimes called  GND (ground) but this wire should not be connected to the electrical safety ground, except at the Modbus master device.

In situations where the polarity labeling is confusing, a good rule of thumb is to ignore the letters and connect to and + to +. If all the communications setting, such as baud rate and address, are configured properly but you’re still not communicating, try swapping the two data wires. Reversing the wires will not damage the device.

When the connection is working, the meter’s Com LED will flash green when the Modbus master device transmits a message. Refer to the Modbus Communication Diagnostics section in the WattNode Modbus meter manual for additional information.