What is this? – well when you send us postal mail, like those regular envelopes mail that you received and sent? well all of them goes into a machine that scan and defaced the stamp prevent re-use. Well the machine is very rough, it will crush anything in the envelope flat. Well sending the a pencil or something like that in the envelope most likely will get crushed by the sorting machine.
So there is a special stamps call “non-machinable”, meaning with this stamp, it would required a human to hand sort this mail, it will not go through a machine and thus will keep the content inside in good shape like a pen pencil etc…