ESP8266 Adapter Board is a breakout or support module designed to make it easier to connect and use ESP8266 Wi-Fi modules, especially the popular ESP-01 ,ESP-07,ESP-08 ,ESP-12v with other systems like Arduino, USB-to-serial adapters, or breadboards.
VCC:3.3V Power (from adapter regulator)
GND:Ground
TX:Transmit (to RX of host MCU)
RX:Receive (from TX of host MCU)
CH_PD / EN:Must be pulled HIGH (adapter usually handles this)
GPIO0:LOW to flash, HIGH to run
GPIO2:Must be HIGH during boot