All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah and that Muhammad 
  is His slave and Messenger. 
Imaam An-Nawawi 
  explained this hadeeth, which was narrated by Imaam Muslim, and clarified why the Prophet 
  compared them to sheets of paper. He 
  wrote:
“The word sesame refers to the well-known sesame from which sesame oil is extracted. Imaam Abu As-Saʻaadaat Al-Mubaarak ibn Muhammad ibn ʻAbd Al-Kareem Al-Jazari, who is known as Ibn Al-Atheer
 said, 'What it means – and Allaah Knows best – is that if the branches of sesame are gathered and left in the sun so that their seeds may be collected, they turn thin and black as if they have been burned, so these people are likened to them.' The hadeeth also reads, 'then they will emerge like sheets of paper.' They are likened to sheets of paper because they will be so white after they wash themselves and all the blackness that was on them is removed.” [Sharh Muslim] 
Allaah knows best.