What does P2P (peer-to-peer) mean?

A network model where every participant is an equal peer holding the same data and talking directly to others, with no central server coordinating them.

A blockchain is a P2P network of nodes, each an equal that compares its copy of the ledger with its peers. The low barrier to running a node is what keeps the network open and decentralized.

Go deeper:

• Peer-to-peer — Wikipedia — contrasts the peer-to-peer mesh against the client-server model.