What is a bone folder?

I wrote an email to manufacturer of the program kit I am going to order.. Part of the instructions she gave me were about making a crease in the paper.. she wrote...

'The printing paper is 8.5 x 11 cardstock. It is not scored, so you will need to fold it in half after printing and maybe use a bone folder to get a nice crease (but isn't necessary).'

So what the heck is a bone folder? anyone know. Or does anyone have any suggestions on how to make a nice crease? I always screw this up and make the page lopsided.

TIA

Oh I have a paper cutter.. I wonder it they sell a scoring attachment.

Thanks for the info..

Does anyone know what a bone cutter is?

here you go munchkin ....
bone folder