Think of it as (very crudley) an RFID tag, similar to those in wireless payment cards.
It's main purpose is ofcourse the "beam" feature, which allows you to wireless transmit items from S3 to S3 (or NFC to NFC compatible items).
You can't pay for things with it yet - VISA is using the Olympic Athletes in the Olympic Village to test out the app so it's a closed beta effectivley - you simply add in your card details, go to a wireless payment terminal and then tap to pay. The system picks up the signal from the phone and then uses the payment information encdrypted in it to process your payment.
Here's how to use Beam: How to use S Beam on your Samsung Galaxy S3 | Reviews | CNET UK