The characters literally combine to mean “speak one, not two.” This powerfully conveys the idea of finality and singularity. If a person says “one,” there is no alternative, no “two” that follows. Their statement is the single, final word on the matter, implying that their decision is non-negotiable and their promise is absolute.
`说一不二` is frequently used to describe a person's character traits. Its connotation can shift from positive to negative based entirely on the situation.