In this indicator, we compare the market capitalisation of Bitcoin(in gold colour) with the market cap of all the other cryptocurrencies (in red), and the market cap of the whole cryptocoin market (the green histogram).
As you may be able to see, when Bitcoin (gold colour) crosses over the Others (red colour) in the indicator, it signifies a swing in popularity towards Bitcoin, and it appears to have predicted some of the recent large rallies (blue Vertical lines) and pull-backs (red vertical lines)
Like all indicators, it is not perfect, and you should never rely on one indicator in any situation.