Synonyms are words that are similar, or have a related meaning, to another word.
For example, synonyms for the word famous include well-known, renowned, celebrated, famed, eminent, illustrious, distinguished, noted, notorious.
1. It is important to remember that when you substitute one word for another, the meaning of the sentence will be slightly altered.
The famous authors recently published their third book.
The well-known authors recently published their third book.
In this case the degree to which the authors are known changes when we change famous to well-known.
2. If you choose the wrong synonym the meaning of the sentence could be altered in such a way that the original meaning is lost.
The eminent author recently published her third book.
The notorious author recently published her third book.
In this case while the author may be famous, the character of the author varies considerably. This changes the meaning of the sentence.