سَافِلِيْنَ

A word in the Arabic language will either be 1) a verb or 2) a noun or 3) a particle – سَافِلِيْنَ is a noun

A noun may either be 1) Definite or indefinite 2) Masculine or feminine 3) Singular, dual or plural 4) Nominative, accusative or genitive

سَافِلُوْنَ is the natural, nominative (مَرْفُوْعٌ) state of this indefinite noun and it changes to its genitive (مَجْرُوْرٌ) state سَافِلِيْنَ due to: 1) a preposition that precedes it or 2) a possessed noun that precedes it or 3) it’s a noun of dependent declension a) نَعْتٌ / صِفَطٌ adjectives b) التَّوْكِيْدُ emphatic c) المَعْطُوْفٌ conjunction d) البَدَلُ synonym

Those who are low

To be low, mean, despicable, vile, base, inferior.

Surah/Below is the only verse in the holy Quran containing the genitive nounسَافِلِيْنَ
Verse
95-5Then We reduced him to the lowest of the lowثُمَّ رَدَدْنَاهُ أَسْفَلَ سَافِلِيْنَ