In the following formula what does the -- mean?
=sumproduct(--(a2:a10=d2),--(b2:b10=e2))

the spreadsheet is:

a b c d e
1 Item Qty
2 pen 5 pen 10
3 pencil 9
4 binder 9
5 pen 6
6 pen 15
7 binder 12
8 pencil 10
9 binder 6
10 pen 11

Harvey Mandel