This is one of the shortened multiplication formulas.
As can be seen, this formula can be used when there is a multiplication between the sum of two particular elements and the subtraction between the two elements.
Instead of presenting them as a multiplication of sum and subtraction, it can be written and it expresses exactly the same thing. In the same way, if such an expression representing the subtraction of two squared numbers is presented to you, you can write it like this:
Pay attention: the formula works both in non-algebraic expressions and in expressions that combine unknowns and numbers.