Sum-Of-Products(SOP) the form of Boolean expression. As you might suspect, a Sum-Of-Products Boolean expression is literally a set of Boolean terms added (summed) together, each term being a multiplicative (product) combination of Boolean variables. An example of an SOP expression would be something like this: ABC + BC + DF, the sum of products "ABC," "BC," and "DF." https://www.allaboutcircuits.com/vol_4/chpt_7/9.html
In your case SOP expression evaluates to a constant (for example: a + ~a =1).