deltaE is a metric, i.e. a way of determining how close two colours are. it is computed by putting together the values of hue, chromaticity and lightness, taking into account how humans perceive colours. the formula used by art (which is a standard one, deltaE 2000) has three parameters that control the weight (i.e. the importance) of each of the three components (h, c and l) in the deltaE computation. this is what the bottom slider controls: the higher the weight, the more the particular component will have an influence on the result. so for instance, if you want a mask that considers all colours that are close to the picked one in terms of hue and chromaticity, regardless of their lightness, just move the lightness weight all the way to the left. you will see, as a visual clue, that the lightness slider will fade away, which tries to suggest that its importance is being reduced.
I hope this clarifies somehow. if not, there’s Wikipedia 
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