The average is calculated by taking into account monthly/weekly/daily highs and average them out.
The script also paints fib levels based on the values of the upper and the lower band.
While this was designed with a cryptocurrency market in mind (no 'close' values per-se) it holds with traditional markets as well.
In true TradingView spirit, the author of this script has published it open-source, so traders can understand and verify it. Cheers to the author! You may use it for free, but reuse of this code in a publication is governed by House Rules. You can favorite it to use it on a chart.