Occasionally, you will need to move files to your website (e.g. new modules or themes).

FTP stands for File Transfer Protocol. It is a common way to move files, modules, themes, etc. to a remote website. FTP is not used to edit the contents of your website.

In many web browsers, you can type: ftp:username:password@ftp.example.com (where username is your user name at your site, password is your password), example.com is your site's domain name), which will allow you to drag and drop files from your computer to the remote server.
For more information on tools that you can use to transfer files to your website, see Development tools

It is unlikely (and inadvisable) for your FTP username and password to be identical to your Drupal username and password! They are probably different. FTP account login happens at a network level below the Drupal CMS. Your FTP credentials are probably supplied directly from your web host, or managed through your web hosts "Account Control Panel" somehow. Check your web hosts help documentation (or read the emails they sent you when you set it up) for more information.
Keeping these two types of accounts separate is more secure, even though it's more to remember.