To paraphrase means to write text written by someone else in your own words. Why would you do this? Well, if the original text is short, clear, and easy to understand you probably wouldn’t. It would be better (and easier) to quote the original text.
You might paraphrase if you want to
There seems to be some confusion about paraphrasing and many teachers and sites may advise you to change a few words (perhaps by using a thesaurus or dictionary) and to change the structure of the original text. You may have to do this but it is a mechanical approach and you risk writing something which is not as clear as the original or, worse, distorting the original sense. If English is not your first language you should be very wary of using a thesaurus or a synonym dictionary. Words and phrases very rarely have exact synonyms; different words with similar meanings usually have different collocates and connotations.
Rather than adopting this method you should carefully read the text you want to paraphrase and think about the meaning. Write something down in your own words which you think captures the essence of what the writer’s text meant. Then put it aside. Come back to it later and rewrite it if necessary. Finally, compare yours with the original and check that you have covered the main points of the the original text and that you haven’t copied any important clauses or long phrases.
By adopting this approach you you are more likely to have a greater understanding of the text you want to paraphrase, you are more likely to remember the ideas in the text you read, and you will be better able to critically discuss these ideas in your own writing.
Note that a paraphrase also requires a citation. Even though you have written the paraphrase mainly in your own words, the idea came from someone else and requires acknowledgement. Furthermore, a paraphrase in academic writing normally requires some kind of introduction in the form of a signal phrase.