Let's use a scenario:
You have 5 bucks (positive.)
You owe me 5 bucks (negative.)
But if I said "You do not owe me 5 bucks," the amount of money you would have would be positive again. The double negative makes the statement positive.
Similarly, the double negative with a number makes it positive:
5 is positive.
-5 is negative.
-(-5) would be positive again.
