Many people come to Joomla! in order to use a site that is already up and running. In this case, you will need to know something about the site and where the Articles are that you are going to work with. This will vary a great deal from site to site, so is not covered here (obviously!) But there is background information about usernames and how to login.
Joomla! allows content to be created and edited by visitors who have a username (sometimes called a login) with appropriate permissions. The usernames are created, and the permissions defined, by the person who looks after the web site, usually called the Administrator.
If you are using an existing site, you will need to make sure that you have been given a username with the right permissions for what you are going to do. So you will need a username in order to be able to add and edit articles, and thus be able to use this hands-on Manual.
There are separate boxes for entering the username and password.
You are likely to be able to see more Menus and other content on the site that you could as an ordinary visitor. If you have the right permissions, you should also be able to edit some of the content.