«نَفْسِي الْفِدَاءُ لِثَغْرٍ رَاقَ مَبْسِمُهُ»
May my life be a ransom for a mouth whose smile so charms me.
Nafsī al-fidāʾ is conventional poetic hyperbole for affection, not the description of an actual sacrifice. Rāqa means to please or delight; mabsim refers to the smiling mouth or the place of a smile.
The line establishes the smile as the central image before the succeeding comparisons unfold.