According to Dictionary.com, the prefix "re" indicates repetition and is generally used to show that an action is being completed a second time. As with many English prefixes, "re" is rooted in Latin. It is most commonly applied to verbs.
"Rereading" a book implies that the speaker has already read the book, and "rewriting" a document means that the document has already been written once. However, "re" is sometimes used on words with roots that cannot be used alone. A user can "load" or "reload" a website, and it's possible to "refresh" a website. However, "fresh" is not a verb, so it's not possible to "fresh" a website.