The middle terms add to zero when they have the same value and different signs, like
(x + 5)(x - 5) = (x)(x) + x(-5) + 5(x) + 5(-5)
When we find the middle terms they are
5(x) and x(-5)
5(x) = 5x
x(-5) = -5x
When we add them 5x + -5x the answer will be 0, because 5 + -5 = 0
So the middle term in the product of sum and difference is 0
(x + 5) and (x - 5) are sum and difference,
then the middle term in their product = 0