This is a plug-in for Arduino IDE, allowing it to "understand" the ESP8266 board.
- In your Arduino IDE, go to File> Preferences
- Enter http://arduino.esp8266.com/stable/package_esp8266com_index.json into the
"Additional Boards Manager URLs" field as shown in the figure below. Then, click the "OK" button
- Note: if you already have the ESP32 boards URL, you can separate the URLs with a comma as follows:
https://dl.espressif.com/dl/package_esp32_index.json, http://arduino.esp8266.com/stable/package_esp8266com_index.json
- Open the Boards Manager. Go to Tools > Board > Boards Manager...
- Search for ESP8266 and press install button for the "ESP8266 by ESP8266 Community":
- That's it. It should be installed after a few seconds.