What is X-UA-Compatible when it references IE=edge,chrome=1?

I just saw this meta tag in the wild and was wondering it does and why it's used?

<meta http-equiv="X-UA-Compatible" content="IE=edge,chrome=1" />

Reading the documentation on About.com, it says:

"IE=edge" tells Internet Explorer to use the highest mode available to that version of IE. Internet Explorer 8 can support up to IE8 modes, IE9 can support IE9 modes and so on.

But what about the chrome=1 tag?

It's for Google's Chrome Frame browser add-on.

ChromeFrame can be installed on various versions of IE (especially handy for older versions that don't play nicely with modern web features). It essentially runs the chrome browser inside of IE.

In the case of the meta tag, IE should run in standards mode (most current edition "Edge") - and activate chrome frames if it exists.

I usually do some conditional browser stuff for older versions of IE, allowing the user to install the add-on as an option.

More here : chrome frame API

(Keep in mind that Google Chrome Frame is no longer supported)

Also, starting in IE11 "edge" mode, which used to be experimental, is now the "preferred" mode - see Compatibility changes in IE11 Preview

Additionally, "document modes" are generally being deprecated in IE11 all together, so you should probably try to stop depending on them.