About using socket, io for socket connection in webrtc

I'm working on peer-to-peer video chat and I'm following Google Codelab to learn. I just read the theoretical part about webrtc from the html 5 rock site, so I don't know much about the encoding part of the socket connection.

The link to the codelab is https://codelabs.developers.google.com/codelabs/webrtc-web/#2, please visit step 05.

And I don't understand the following in the "main.js" file.

socket.on('message', function(message) {
  console.log('Client received message:', message);
  if (message === 'got user media') {
    maybeStart();
  } else if (message.type === 'offer') {
    if (!isInitiator && !isStarted) {
      maybeStart();
    }
    pc.setRemoteDescription(new RTCSessionDescription(message));
    doAnswer();
  } else if (message.type === 'answer' && isStarted) {
    pc.setRemoteDescription(new RTCSessionDescription(message));
  } else if (message.type === 'candidate' && isStarted) {
    var candidate = new RTCIceCandidate({
      sdpMLineIndex: message.label,
      candidate: message.candidate
    });
    pc.addIceCandidate(candidate);
  } else if (message === 'bye' && isStarted) {
    handleRemoteHangup();
  }
});

So my question is

1) What is message.type === 'offer' and what kind of string is offered? Same goes for "answer" and "candidate".

Can you tell me how this code works?