A panel is the combination of a screen, and some kind of input method, attached to the machine, that allows you to easily do things like move the carriage, start a file, monitor its status, etc.
In order to use a panel with a Smoothieboard, you need to wire it to the board, and to modify the configuration file so the board knows it’s talking to a panel.
There are different types of panels supported by Smoothieboard. To learn how to wire and configure for your specific panel, please read the Panel page.