The Super Eraser is attached to the end of a giant pencil. It is pink, much like a stereotypical eraser.
The Super Eraser, when used, can erase scenes from Poptropica - or rather, steal things from the scenes, leaving the scenes blank. How this is done is not explained.
Arthur Eraser originally wielded the pencil, using it to attempt to erase parts of scenes, stealing them for his own private island. Using the eraser, he erased quite a bit of Poptropica - however, with your pencil, you can draw things back in.
Arthur gradually erases things throughout the book using the eraser. At the end of the book, when you catch Arthur, the eraser - and the pencil attached - are no longer in his possession. It is undisclosed where the Super Eraser went.