Can anyone explain the -- in layman's terms?

[Celt]

I was trying to use a SUMIF function with 2 criteria (i.e. look for specific data in 2 columns and if true sum the number in the 3rd column).

Someone gave me this formula:
=SUMPRODUCT(--($C$5:$C$21=$B$2),--($D$5:$D$21=$A4),($E$5:$E$21))

I understand the SUMPRODUCT function. My search criteria (B2 and A4) are non numeric, so the function treats them as 0. I just don't understand what the "--" is doing in this function.

Any help is greatly appreciated!!

[Domenic]

Have a look at the following link...

http://www.mcgimpsey.com/excel/formulae/doubleneg.html

Hope this helps!