Which language is this (WordPress)?

I’m debugging the WordPress-Code and in the file media-template.php I found code that looks like this:

<# if ( ! _.isEmpty( data.model.src ) ) {
if ( isYouTube ) { #>
<source src="{{ data.model.src }}" type="video/youtube" />

Can someone give me a hint what language this is? This is not PHP. How will the variable data.model.src be filled?

This full source code can be found on GitHub here: https://github.com/WordPress/WordPress/blob/master/wp-includes/media-template.php

>Solution :

Thats Backbone JS
You can lookup whats actually happening with this here.