It measures the angle of a Moving Average in order to filter out ranging markets.
The idea is to only enter a trend following trade if the slope is steep enough.
In order to create this indicator, I used a strategy script from bennef called "Trend Angle BF" and slightly modified it to transform it from an entry/exit indicator to a filter.
The calculations are based on Evergrets "Jurik MA" and angle calculation by KyJ.
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.