They cancel out each other
Imagine if you were multiplying
-2 * -3
you would get 6
if you did
-2 * 3
you would get -6 because there isn't another - to cancel it out
I learned it that when you have a negative before a negative, put parenthesis around the second number with the negative, so it looks like this
-(-2)
if you look at the -(- it looks like a plus sign
that probably doesn't make much sense but we cancelling it out is just a rule, we don't really know why we do it
