Then it plots the average of those 20 ROC's as a yellow filled area so as to make it easier to see where the balance (or "tide") of the waves are located.
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.