When you log in to your cpanel you are asked for a user name and password and that username is what it is asked for when setting this up.
Lets say you log in to your cpanel with the username "forum".
From what I understand, all the databases that the users set up on a server are stored at the same place on the server, so the server needs some way to know that account "forum" are allowed to access this specific database, therefor you will need to add the cpanel user name (in this case forum) before the database name and the specific user name you created for that database.
So this is what it should look like,
Host name: localhost
MySQL User Name: forum_admin12
MySQL Password: passfows
MySQL Database Name: forum_root