I looked though an job ad and found the words AR deliquency but I have no real idea what it is about, could you help ?

That's collections.

From the wording, it sounds like you'd be the one calling the customers whose accounts are past due.

... but it's possible that it is a collection agency.

Either way, it's calling people who owe money.

A/R stands for Account Receivable. Delinquency (in this case) means past due. So it refers to funds due a company that are past due.

Frankly, if you couldn't figure out what it means, then you probably aren't qualified for that job.