The highest and lowest value of the source over the lookback period are used to determine the highest and lowest point - the green and red lines at the top and bottom of the bands. This region is then mathematically split into three, such that the source (and its optional moving average line) can be defined as being in the top third, the middle or the bottom third.
Applications for this could be in risk management where you may wish to take on a larger position size when a certain indicator is in the top third, or decide that you want to enter / leave positions when the source crosses in / out of the extreme points.
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.