In math, like algebra and calculus, a product is special when it is very common and worth knowing.
Some examples area:
(x + y) = ax + ay (Distibutive Law)
(x + y)(x − y) = x2 − y2 (Difference of 2 squares)
(x + y)2 = x2 + 2xy + y2 (Square of a sum)
(x − y)2 = x2 − 2xy + y2 (Square of a difference)