SFTP stands for SSH File Transfer Protocol. It is an extension of the SSH protocol that allows secure file transfer, management, and access.
If the software you are uploading with supports this feature, you can use it to upload files to your account over SFTP on port 22.
For SFTP host, you need to use the hostname of the server where your account is hosted. This information can be found in the left pane of the Control Panel of the hosting account under Server name (your_server.com on the screenshot).
Since SFTP requires SSH access, and SSH access is available only to the main Control Panel user, only the main Control Panel user is able to use SFTP to upload files. You can see the name of your main user in the left pane of the Control Panel of the hosting account under "Username" (your_username on the screenshot).
In order to upload content over SFTP, SSH access to the account needs to be enabled. You can enable SSH access for the main Control Panel user through the SSH Access section in the Control Panel of the account. Detailed instructions are available in the following article:
When connecting over SFTP, you can either authenticate with the username and password of the main Control Panel user, or set up key-based authentication. Detailed instructions on how to set up key-based authentication are available in the following article:
Key-based SSH authentication