فَكُّ رَقَبَةٍ
Sentence structure
An understood topic is supplied by the question. فَكُّ is its nominative predicate and is annexed to genitive رَقَبَةٍ.
Ayah summary
It is to free a person from slavery or debt.
فَكُّ — (It is) freeing
Grammar: A definite-by-annexation masculine verbal noun, nominative predicate of an understood topic and annexed to the next noun.
رَقَبَةٍ — a neck
Grammar: An indefinite feminine-singular genitive complement of فَكُّ.