The multimedia submenu keeps all of your favorite applications such as the sound player, the video player etc.
What to do
This option keeps shortcuts to some of the most frequently used applications, System administration, multimedia, Documentations, graphics utilities, Internet, etc.
Administer your system, you can add new software, change your password, configure KDE, manage your files your users etc.
Look at the picture to the left, these are the shortcuts on the menu.
Another important item on this menu that you haven't seen is the search files. The search files option is one of the best tools you have on the desktop.
As you've already noticed, documentations are everywhere. Many of these links may be removed once you know your way around.
This is basically the configuration (control center), also available on the KDE menu, just like on the task bar.
The application menu has other administrative utilities, such as accessibility, file manager, archiving and backups, publishing and text editors.
If you need access to other parts of the hard drive, then use the application submenu to access the file manager. This file manager runs as root.
The last part of the KDE menu holds some of the most recently executed applications.
The right click menu
By moving your mouse to the middle of the desktop and right clicking on it, you can see a menu with all sorts of commands.
From here you have access to the run command, create new directory, configure your desktop, and lock the screen or logout.
On your main desktop also appears an icon for your home folder; by default you have read write access in your home folder.
To copy files use drag & drop wherever you have permission; or use copy paste instead.