This is a plug-in for Arduino IDE, allowing it to "understand" the ESP8266 board.

1. In your Arduino IDE, go to File> Preferences
2. 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
3. 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
4. Open the Boards Manager. Go to Tools > Board > Boards Manager…
5. Search for ESP8266 and press install button for the “ESP8266 by ESP8266 Community“:
6. That’s it. It should be installed after a few seconds.