هُمُ

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

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

هُمُ is a suffix (a word added to the back of a word), a possessive pronoun of the third person, masculine, plural

هُمُ is the attached pronoun هُم and takes a dammah due to the definite noun following it in a sentence

Means ‘them’ when used as an affix after a verb or preposition
Means ‘their’ when used as an affix after a noun

Grammatical analysis of هُمُ 
ضَمِيْرٌ مُتَّصِلٌ مَبْنِيٌّ عَلَى السُّكُوْنِ 
مَعْرِفَةٌجَمْعٌمُذَكَّرٌمَنْصُوْبٌمَجْرُوْرٌهماَلْغَائِبُضَمِيْرٌمَبْنِيٌّاِسْمٌضَمِيْرٌ مُتَّصِلٌ
DefinitePluralMasculineAccusativeGenitiveRootThemPronounFixedNounAttached pronoun
       Their    
Surah/Below are the 2+ verses in the holy Quran containing the plural, masculine suffixهُمُ
Verse
1
1
Not were those who disbelieved from the people of the book and the polytheistsلَمْ يَكُنِ الَّذِيْنَ كَفَرُوْا مِنْ أَهْلِ الْكِتَابِ وَالْمُشْرِكِيْنَ مُنْفَكِّيْنَ
98-1to be freed until the clear evidence comes to themحَتَّى تَأْتِيَهُمُ الْبَيِّنَةُ
98-4And those who were given the book differed not until after clear evidence came to themوَمَا تَفَرَّقَ الَّذِيْنَ أُوْتُوْا الْكِتَابَ إِلَّا مِنْ بَعْدِ مَا جَائَتْهُمُ الْبَيِّنَةُ