It is used to send files from one computer to another over the Internet, and is used in preference to sending a large message via email. For purposes of this discussion, let's assume that your login name is 'fred', and your password is 'password'. It's the way you get stuff from the outside world. It could be to your friend across the street, or to someone in China - doesn't matter. Well, glad you asked.
Most sites have an ftp server.
Once on our ftp server, you'll see a folder marked 'pub'. Click on it, and you can read a quick explanation of what Eudora is. This is a copy of the Eudora mail-retrieval program. Your browser will prompt you to 'Save the file', and then prompt you for a place to put it. UPloading is just the opposite of DOWNloading.