The RS485 Board (3.3V) is an accessory board used for adding the RS485 transceiver to your application board.
The RS485 Board (3.3V) features a SP3485 onboard, integrates pinheaders and connectors to support multiple communication interfaces. It is designed to use 3.3V power supply.
More information:
https://www.waveshare.com/rs485-board-3.3v.htm