P2P is a feature on most exchanges which allows people to trade directly with another party, without having to put up a sell it buy order and wait for it be taken or buy at the market value, where the exchange would match them up with a seller or buyer.
Decentralized and Centralized platforms alike can have a P2P system, the difference is that in a DEX, they do not hold your funds and you would be trading directly from your personal wallet to the buyers address. This is not the case in a CEX where you need to find an address which is owned by the exchange and then trade directly from their with another peer.