A/R = "Accounts Receivable". It's a bookkeeping term referring to open invoices (money owed to the company.)

In this context, it means that there's some bookkeeping involved for this job.
Accounts Receivable. It is money that is owed to a company for services or goods provided.

You may also run into A/P which stands for Accounts Payable. That is money owed by the company to others for receiving services or goods.
A/R experience stands for "Accounts Receivable". Essentially collecting money owed to the company.
if you don't know, then you don't have the experience. Plain and simple.