And give orphans their properties, and do not substitute the bad for the good. And do not consume their properties by combining them with yours, for that would be a serious sin.*
Who are the orphans according this verse.
Who counts as an orphan is answered in When does someone stop being an orphan? It is someone who:
is a paternal orphan (i.e., their father has died), see Why are orphans considered orphans when their father dies and not when both their parents die? for more about this; and
has not reached adulthood, normally regarded as reaching puberty.
A tafsir by Asbab Al-Nuzul by Al-Wahidi describes this history of this ayah:
Muqatil and al-Kalbi said: “This was revealed about a man from Ghatafan who had in his possession an abundant fortune which belonged to his orphaned nephew. When this orphan reached the age of puberty he claimed this fortune but his uncle refused to give it to him. As a result they took their dispute to the Messenger of Allah, Allah bless him and give him peace, and then this verse was revealed. When the uncle heard this verse he said: ‘We obey Allah and we obey His Messenger; we seek refuge in Allah from the great peril’, and he handed the wealth over to his nephew.
(There's other tafsir at altafsir.com which tell a similar story.)