XI Adapter is a package you install on Integration Server so you can interact with business partners using SAP PI/PO Servers.

PI/PO Servers enable business partners to send each other MessageObjects. MessageObjects consist of a message header and a payload. The payload contains the task data the business partner is sending such as a purchase order or an invoice. For example, if you were a travel agency, you might use a PI/PO Server to send MessageObjects that request flight bookings to an airline company. If you were an airline company, you might use a PI/PO Server to send MessageObjects that request payment to travel agencies for flights you booked.