p2p has a lot of different meanings out of context, but I believe this must refer to procure-to-pay. In the software industry, it is part of the procurement process. Systems like this allow visibility and control of transactions throughout the transaction lifecycle, so it is typically found when an organization wants to centralize its procurement function.
p2p systems link certain functions back to the finance department such as supply management, purchase orders, and receiving. To implement a p2p system, you need to have a good understanding of what the current processes are as there will have to be some changes to insert finance into the loop.