This is the 2639th most frequent German word.
dein
"Dein" means "your" (informal, singular).
In this sentence, 'dein' is used as a possessive pronoun to indicate ownership or possession related to the second person singular subject.
Ist das dein Buch?
Is this your book?
Here, 'dein' again serves as a possessive pronoun, describing something that belongs to the second person singular audience.
Dein Hund läuft im Garten.
Your dog is running in the garden.
In this case, 'dein' modifies 'Geschenk' indicating possession of the gift by the second person singular.