What is meant by synonym?
A synonym is a word that has a very similar meaning to some other word. For example, the words large and big are synonyms. Small and little are synonyms. But large and small are antonyms.
A synonym of a synonym is an alternate word for one of the original word's synonyms: A synonym for "abundance" is "plenty." In that sentence the word "abundance" is the original, and "plenty" is the synonym. It follows that a synonym of the synonym is one of the synonyms for the word, "plenty;" for example, "copiousness."