It is measured in USD (converting the M2 of each of the countries respective currencies and virtually converting them into USD)
This is less than the global liquidity as it does not include the countries' assets in other currencies (on their balance sheets), it only focuses on the monetary supply of each of the countries own currencies.
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.