Takes the first candle and checks if its a green candle (close > open), and adds the to a total (which starts at 0 at the start of the day).
Subtracts it if the candles in red.
Continues to do this along with all the candles.
The is also multiplied by the difference between open and close so that large candles with large have more weight and move the indicator more.
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.