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)