The Quantile Bands MT4 Indicator attempts to divide the market into equal pieces using quantiles. Those quantiles are displayed by the two outer bands. Using the standard settings of the indicator, those bands show the 10% and the 90% quantiles.
Using for example the 10% quantile in this context means that 10% of all values of the specified period are lower than the current value of the lower outer band. The middle band is not the slope of the middle quantile, but a combination of the two outer bands.