Unique to each individual.

We all have fingerprints however they are unique. Even identical twins have different fingerprints. However nobody knew this 1400 years ago. But it was mentioned in the Quran that on Resurrection Day Allah will recreate humans with all details even their fingertips. Today we know that those contain fingerprints which are unique to each individual.

Quran 75:4
Yes indeed; We are Able to reconstruct his fingertips. 

٤ بَلَىٰ قَادِرِينَ عَلَىٰ أَنْ نُسَوِّيَ بَنَانَهُ

On Resurrection Day Allah will reconstruct our bodies to the smallest detail even our unique fingerprints.

How could an illiterate man who lived 1400 years ago have known that fingertips have something unique?

The Quran (Koran, the book of Islam) contains scientific knowledge that could not have been known 1400 years ago. It ranges from basic arithmetics to the most advanced topics in astrophysics. You are invited to go through those miracles and judge for yourself.