in the old system (centralization and banks) when i want to send you money i go to the bank and give them my money and ask them to send it to you and you go to them to receive the money that i sent you and we pay them a fee.
in the new system (decentralized and bitcoin) you and i are peers on a network of peers like us. when i want to send you money i directly send it to you without any mad in the middle through this network that is shared among all the peers called the bitcoin network and store the record of it on this public ledger called blockchain.
Peer to peer which is usually shortened as P2P means sending a transaction from one person to another, in such case the both party as agreed on amount and the method of payment.