Not really sure what answer you are looking for here, as the functions are quite well described in the manual. What makes this specific piece of expression look a bit nasty is that the the dollar expansion input is a string.
Basically what happens here is that the first Concat() produces a AND eprataed string, which then becomes the IF statement's condition. Followingly the hash128() result then becomes the output of the if statement if the condition is true. In both Concat()'s the string is based on an iteration over $Field.
=if( concat(not IsNull($Field),'AND') ,
hash128( concat($Field,',') )
)
concat([{set_expression}] [ distinct ] [ total [<fld {, fld}>]] expression[, delimiter[, sort_weight]])
Returns the aggregated string concatenation of all values of expression iterated over the chart dimension(s). Each value may be separated by the string found in delimiter. The order of concatenation may be determined by sort-weight. Sort-weight should return a numeric value where the lowest value will render the item to be sorted first.
hash128( expression {, expression} )
Returns a 128-bit hash of the combined input expression values. The result is a string.
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