Thread: Sumproduct with conditions?

I was looking through one of my old sheets and came across this:

=SUMPRODUCT(--(I4:I1151=3),--(J4:J1151=1))

I can't remember for the life of me how I figured out that would count the occurrences of the two conditions. Any idea what the "--" does? If I take them out excel doesn't count any instances where the criteria are met.

Thanks in advance.