سَوْدَاءُ وَاضِحَةُ الْجَبِينِ كَمُقْلَةِ الظَّبْيِ الْغَرِيرِ
Dark in complexion, with a bright forehead, like the beautiful dark eye of a young gazelle.
The verse compares a dark feature with the darkness of a gazelle's eye, a conventional image of beauty in Arabic poetry. The model encourages the listener to appreciate the attractiveness of the feature.
Its primary purpose is not merely to measure how dark the feature is, but to present it as beautiful: tazyīn.