What is the meaning of the abrogated and the allegorical verses, and what is the meaning of "they are the Mother of the Book"?

The definitive (muhkam) among the verses of the Book is its foundation, upon which judgments, permissible acts (halal), and forbidden acts (haram) are based. Anything that contradicts it is referred back to it. It is clear in meaning and evident in its indication. Whereas the ambiguous (mutashabih) is that whose meaning is not clear or whose indication is not apparent.