It defines downtrend as following:
Lowest price of last 2 days is equal or lower then lowest price of last 100 days.
Values 2 and 100 are customizable.
Uptrend analogically with highs.
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.