These are just conditional statements that are read by internet explorer, and allow web devs to get around the plethora of issues developing for IE brings.
Ironically, IE10 and above drops support for these entirely so they are really only useful for retro coding for old browsers, i,.e when using HTML5 shivs such as
<!--[if lt IE 9]>
<script src="dist/html5shiv.js"></script>
<![endif]-->
How do they work.. IE (below 10) just detects them as part of its rendering engine, and other browsers just treat as comments
Here some info about them being removed, and a link to other resources if you want to look more into it, or get better examples
http://www.therailsview.com/2012/02/internet-explorer-10-to-no-longer-support-conditional-comments/