Consider a situation where you have a process that manages purchases. Your suppliers can access the system through some kind of interface, perhaps a Web front end. You create an instance in your process by sending a purchase order to a supplier. Your process will expect the seller to return an acknowledgment for the order, stating if the order is accepted or not, so the process waits for this acknowledgment.
For the process to continue, the acknowledgment must be routed to the same instance that originated the purchase order. However, your supplier does not have the instance ID. So then the question is: How can you route the acknowledgment from the supplier to the correct instance? You could send the instance ID to your supplier, but this ID has no meaning to the supplier, and the supplier may not even have a way of storing it. In fact, the instance ID has no meaning anywhere outside the process.
In this situation, you can define a correlation set that correlates a business token to a given instance. A business token is simply a value or set of values that has business meaning and is also unique to each instance. In this example, you can use a purchase order number as a business token, but you can't use the date or amount of the purchase, because these values are not unique to a given order.