You need to know the ftp address of the server that hosts your page, together with your username and password.
For example, I've got a page on www.webtribe.net. The ftp address is ftp.webtribe.net. I connect to it using an ftp program. (I use Coffee Cup Free FTP). I tell the ftp program the ftp address to connect to, together with my username (which is the first part of my e-mail address with Webtribe) and password. Once I'm connected, two panels are displayed. One displays files on my PC. The other shows files on 'my bit' of the Webtribe server. I can use the controls to move files from one site to the other.
I keep a copy of my webpage on my PC. If I want to change the website, I amend this page and then upload it to the Webtribe server, where it overwrites the existing file.
If I'd not got a copy of the page on my PC, I could use the ftp program to 'grab' it from the Webtribe site, before amending it and uploading the revised file, as above. (Some web hosts permit users, with suitable ftp software, to edit a file directly on their server but most free services require you to do the editing on your own PC, then upload the revised file).
Although I've made reference to a 'stand alone' FTP program, you might have used the FTP facility which is built into many website creation programs.
Chris