Click at the "Edit" icon at the left side of a username.
Username: the name of the user
Password: The password this user needs to log into the CMS. Click "Generate" to use a random password. (save the generated password on a secure location) A custom password could be used too. This password has to be confirmed in the input field underneath the first one.
Role: The role of a user will determine the acces rights of a user in this CMS. For example, an admin user is able to add other users. An editor or webmaster user is not.