Escrow is provide by a third party service that holds a payment until products or services are delivered. For example, if I had one bit coin for sale and you wanted to buy it, I wouldn't just send you the bitcoin and trust that you will send me my payment. I might require escrow. In this case, you would send payment to a third party escrow service and I would be notified that the payment was being held. I would then send you the bitcion. Once you receive the bitcoin, the escrow service would send me the payment. Escrow is a way to bring in a trusted third party for a transaction.