Are you serious?
They are whip scars. Back in the slavery days, the slaves would get whipped, leaving marks like that. The word 'Nigger' in itself comes from the days of slavery, in case you didn't know. It was the word used to degrade the slaves, for example, white owned businesses would have signs saying "No niggers allowed", etc, etc, etc - this kind of discrimination lasted wayyyy past the days of slavery, even until the mid-1900's.
By having those whip marks on his back, the cover is basically a visual representation of the word 'Nigger', which is what Nas has done since he can't call the album that.