If you use an ordinary pencil – to test out the design of your decoration, for example, or to indicate where you intend to apply colour – your drawing will simply burn away. But if you use a ceramic pencil, the drawing will be visible even after firing.
The core of a ceramic pencil consists of ceramic pigments. You can actually make these pencils yourself, but you can also just buy them from your pottery shop. You use them to draw on clay that has been fired a first time, i.e. bisque fired.
The smoother the surface, the better you will be able to draw. So it might be a good idea to polish the surface you want to draw on before you start. Happy drawing!