English has thousands of words that convey meaning and play a role in communication. With such a huge vocabulary it is not possible that every individual word will have a totally unique meaning. There are many words that have similar meanings to each other - and not only that, we also have words that have opposite meanings.
What does the word PAL remind you of? Does it make you think of a FRIEND, a BUDDY or an ALLY? These words are similar in meaning to PAL and such words are called synonyms. Synonyms help you to pick a suitable word to communicate your message clearly and precisely.
Related to synonyms are antonyms. Words that are opposite in meaning to each other are known as antonyms. In English some antonyms are formed by adding a prefix to a word and these prefixes have meanings that contribute to the opposite meaning of the new word formed.