Adafruit AVR Board

I am a complete noobie here. If the answer to my question is already here thank you to everyone. I just downloaded and installed Arduino 1.6.6. I now have to install Adafruit AVR Board. I have the url for the board but do not know how to install it. I found the place for additional board url is mentioned. But I do not understand how to load the Adafruit board. Could some one provide a video or instruction on this procedure?

What is the Adafruit AVR Board you have?

This is all I have.

https://adafruit.github.io/arduino-board-index/package_adafruit_index.json