FTP - which stands for File Transfer Protocol - lets you upload your website's files to your hosting account (or download those files from your hosting account to your computer). By doing this, you're publishing your Website to the internet to make it visible to visitors.
To use FTP, you'll need an FTP client. There are many different clients you can use, but we recommend FileZilla.
Some website building software - such as Dreamweaver - may also have their own built-in FTP features for uploading your Website.