Hi,
You can get the simple description (in 3 lines) on official site:-
PeerJS wraps the browser's WebRTC implementation to provide a
complete, configurable, and easy-to-use peer-to-peer connection API.
Equipped with nothing but an ID, a peer can create a P2P data or media
stream connection to a remote peer.
this explains the complete thing. But you want more then you can check:-
http://xirsys.com/peerjs/
http://peerjs.com/docs/#api
Please read once and if you have any confusion then let us know.
With Regards,
Rishikesh kumar