wrtc.localVideo is an object property containing the HTML5 <video> object used to show images from local camera. The image of yourself.
If the HTML5 <video> object you intend to use has id="localVideo" you do not need to set wrtc.localVideo, only if the <video> object has some other id.
Property containing HTML5 <video> object.
document.getElementById("localVideo")
Usually you set wrtc.localVideo in wrtc.init(). If you change it later you need to call wrtc.changeMedia() after.
wrtc.init = function() {
…
wrtc.localVideo=document.getElementById("myvideo");
…
};