This is the 27th most frequent Icelandic word.
ár
"Ár" in Icelandic means "year."
Here, 'árin' is the plural form of 'ár' meaning 'year', used as a noun referring to time.
Árin líða hratt.
The years pass quickly.
In this sentence, 'ári' is in the dative singular form of 'ár', denoting the temporal frequency of an action.
Við förum í sund einu sinni á ári.
We go swimming once a year.
Here, 'árinu' is in the dative singular definite form of 'ár', indicating a specific instance within the year.