P2P stands for Peer-2-Peer.
The internet is built on top of the client-server paradigm, where the web browser is the client and the server is the various websites people connect to through the browser. In this paradigm, there is no ability for one web browser to reach another one directly – there must be a web server acting as an intermediary.
An example of this can be a chat service, where messages between two users chatting must pass through a web server.
P2P, on the other hand, is a technology where two clients can communicate directly with each other. The only standardized means for doing that across web browsers is by using WebRTC.
P2P reduces the load on servers, reduces latency of messages and increases privacy. It can also be used to create mesh networks.