This is the 29th most frequent Icelandic word.
þeirra
"Their" (possessive pronoun).
The word 'þeirra' here is a possessive pronoun referring to possession in the third person plural, indicating that the car belongs to 'them'.
Bíll þeirra er blár.
Their car is blue.
'Þeirra' is used here to express possession of the talents by 'them', functioning as a possessive form.
Hæfileikar þeirra í tónlist eru frábærir.
Their talents in music are amazing.
The word 'þeirra' shows that the friends referred to are associated with 'them', serving as a possessive construct.