This is the 268th most frequent Icelandic word.
árin
"Árin" means "the years" in English.
The word 'árin' here is used as the subject of the sentence in its plural nominative form, referring to multiple years.
Árin líða hratt þegar maður hefur gaman.
The years pass quickly when one is having fun.
The word 'árin' is used in the context of summing up an experience, highlighting the properties of multiple specific years represented as a subject.
Ég hef lifað í níu ár hér, og árin hafa verið stórbrotnir.
I have lived here for nine years, and the years have been magnificent.
Here 'árin' is plural and used to denote the agent (years as entities) contributing to the subject's personal growth.