It is tracking Previous candle high and Previous candle low. It is an important script to trail PDH PDL system (and more importantly to create alerts) which is rampantly used by price action traders.
Release Notes: Previous Script has certain restrictions for intraday. That has been fixed here.

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Open-source script

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.

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