-- in sumproduct

Although I've used sumproduct for years in its "normal" application, have just been introduced to a new syntax of it as a replacement for sumifs.

As below is an example, and I understand everything except -- why the double equal characters?  What do they actually signify to the function??  It won't work without them.

=SUMPRODUCT(--(LEFT(B27:B42,3)="xxx"),D27:D42)

Thanks.

 - Mik