You may enter any single word as the hostname - this is just a name for your server, so you can enter something like server1 , or mysneakerserver , and it would be accepted as your server's hostname accordingly. Please do not use any special characters like % or ! or * ^ $ , as these are cannot be accepted for the hostname. Also, please do not enter a blank space (in between the words), as that is not accepted as well.
And as for the root password, this is simply your server's login password. Please enter a password that has at least 12 characters, contain at least 1 upper case, 1 number, and if you are able to include 1 or more special characters into the password (eg: # @ ! $ % & * ), that would ensure a safer and more secure password for your server.