What it is
The AR Agent turns the payment remittances your customers send into posted cash receipts on its own. It takes each remittance in, reads it however it arrives, checks it against your open invoices, and records the payment in your system, without a person typing the numbers in by hand.
It handles what a real AR desk actually receives: check stubs, payment advice, scanned paper, and spreadsheets. Anything it is not sure about is set aside for a person to look at, so nothing wrong slips through to your books.