What does "--" used in Excel function mean?
All the -- does is tell Excel to convert the boolean search results to 0 or 1.
Anytime you need to use some type of logical function in Excel and wish to have the results returned as either “1” or “0” instead of “TRUE” or “FALSE,” simply use double minus signs as part of your formula and you will be well on your way to creating some powerful Excel formulas.
This source does a great job of explaining how to use this feature.