This is a composite volatility index to show percentile of current volatility compared to that of the last 52 bars. As this is a weekly chart (and this script is intended for usage on weekly charts) we can see the yearly percentile rank of volatility.
As shown when volatility is in the lower 25%tile (viewed on weekly) the market is calm and likes to rise; when the volatility is above the 25%tile you can see that the market tends to have larger and 'choppier' moves.
This is /not/ 'just the vix' this takes into consideration the volatility of all major US indexes including the SPX500, Dow 30, Nasdaq 100, and Russel 2000.
Please remember that this is just plotting:( volatility index - lowest(index,52) )/( highest(index,52)-lowest(index,52) ) so for 'yearly percentile' check the weekly chart (52 weeks = 1yr)