It represents something that is there that you don't know the value of. So when you use it as part of an expression even as a counterpart to a term that you kno

It represents something that is there that you don't know the value of. So when you use it as part of an expression even as a counterpart to a term that you know the value of, it "corrupts/interacts" so that the return value of the expression is unknown whereever the expression's return value is dependent on the value that we had chosen to represent with NULL. This is different semantics from say, taking the AND with a term that is not present, which in most conventions would have us substitute the identity value (i.e. TRUE), so that the return value is TRUE if and only if the known value is TRUE; instead, we have that the return value in SQL is NULL.