The FTP account will have access to all files and directory structure under the directory you set up here. You can use any existing directory or create a new one. It depends on what access you want to give this ftp account.
For instance the default will be /public_html/myftpaccount/ and the account.
If you want to give this ftp account access to your public html (your website) structure, then change it to only be /public_html/
If you want to give this ftp account access to everything on your account (same as your main ftp account), then remove all the text, so it will only be /
