Antonyms are words that have opposite meanings, such as 'on' and 'off'. The opposite of 'right' is 'left'. The opposite of 'antonym' is 'synonym'.
Numerous words in the English language have an opposite, but not all of them. If you think of the word 'apple' there isn't an antonym of this word. Dictionaries often add antonyms ~ good, larger dictionaries that is. Getting familiar with new words ~ how to spell them; what they mean; their synonyms and antonyms ~ will add to your knowledge of the English language. If you want to be a writer (of any kind) you'll need to have a good vocabulary. Using an alternative (and maybe more unusual) word will enhance your writing.
Try this 11-plus quiz and see how many antonyms you know.