The English language is host to thousands of words which describe all kinds of things, places and persons. Through correctly using these words people can communicate with one another clearly. Obviously not all words can be unique and there would be some words which are similar and some words which are opposite in meaning. Words with opposite meanings are called ANTONYMS.
GOOD is the opposite of BAD, UGLY is the opposite of BEAUTIFUL and STRONG is the opposite of WEAK. All of these example are antonyms.
Antonyms help to bring contrast into writing or speaking. For instance a book could be a GOOD book or a BAD book. Here, GOOD and BAD represent the opposite. Similarly a girl could look BEAUTIFUL or she can look UGLY. Here again, BEAUTIFUL and UGLY represent the opposite.