A Taskbar is an application, showing what programs you're running. It's mostly located in the bottom of your screen, and most people stretch it over the whole screen. Some people also call it a panel.
Both KDE and Gnome use a taskbar to simplify the interface (User_interface). But unlike in other operating systems you're not limited to one specific taskbar, there's a lot of choices you can run. Especially if you don't run one of the environments but only a WindowManager.
In Debian you'll find following apt-gettable packages which will give you a full-featured taskbar:
Note that some window managers bring along their own taskbar/panel so maybe you won't need one of the above.