When you get a new hosting account you get an account user. These user details can be used to log into cPanel if you have it, but can also be used to connect via an ftp client to upload your sites files.
This always puts me on edge though. If someone gets your details they can’t just access your ftp but can also log in and make any changes that they want to your site.
For this reason I generally set up a new ftp account just to use for ftp access.
Until now I’ve never worked out how to access the root of a site with an ftp account. Generally I just accept access to the public_html folder and leave it at that. But today I finally worked out how to give an ftp account root access to a hosting accounts files.