net2ftp is a web-based FTP client written in PHP. Lets explain this in detail. Web-based means that net2ftp runs on a web server, and that you use a browser (for example Internet Explorer or Mozilla).
FTP is the communication method to access files on a remote computer. You need it to access your files on the account given by your provider to host your homepage. FTP is different from HTTP in that it was made specially to transfer (big) files. If a transfer is interrupted, with FTP you can resume it where it broke off, whereas with HTTP you can not. PHP is a programming language that is used to make dynamic websites.
You can use net2ftp at http://www.net2ftp.com, or you can install it on your own web server and use it from there.