A website is usually a combination of a set of files and a database storing some data for the website. The files and any database are usually located on some server space offered by a hosting company. People do not tend to build a website completely from scratch these days and instead often take a template website implementation (which in most cases is a CMS (Content Management System like WordPress, Joomla, Magento, Drupal, etc.) and then add their business content and change the look and feel of it to suit their brand.
To add to and adjust the data that makes up a website one needs an FTP, SFTP or SSH connection to the server. When connecting to a server they need to know the address of the server which in its rawest form is a unique IP address. The access details will be provided to a website owner by the hosting company when they register with them to get the space (hosting account) for their site.