The command line in Linux is just too powerful a tool to give up entirely, so the KDE gives you access to it in two different ways.

The first way to access the command line is with the Terminal program. This is used to access the command line on a long term basis (in fact, you don't even need to close it). To open it, just select it from the Utilities section of the Application Launcher.

A shortcut method of accessing the command line to enter a single command is with the ALT + F2 key sequence. Just hold down the ALT key on your keyboard and at the same time press the F2 key. In the center of the screen a window will pop up asking you to enter a command. Just type in the command and hit enter, the command will be executed and control returned to the KDE.