Imagine you like to draw pictures in a coloring book. A penciller is someone who draws pictures too, but they don't color them in like you do. Instead, they use a special pencil called a non-photo blue pencil to sketch out the shapes and outlines of the picture. These lines are usually very light and can be erased easily if the penciller makes a mistake or needs to change something.
Once the penciller is done with the sketch, someone else (like an inker or a colorist) will usually go over the lines with ink or color to finish the picture. But the penciller's job is very important because they create the foundation for the entire picture. Without their sketch, there would be nothing for the other artists to work off of.
So, in short, a penciller is like a master sketch artist who draws the basic outlines of a picture before it gets finished by other artists.