It is simply a ratio of asset A to asset B plotted. Usually it is used to look for strength vs a particular index. Since it is a ratio, all the work on it. The default index is NIFTY . You can change it any index/script you want to compare:
1. Script vs Index
2. Index vs Index
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.