Internet Explorer "compatibility mode" is that dreadful page icon, inconveniently placed right next to the refresh button/icon.
What it does is turn IE 8 or higher into an outdated browser, so it can properly display really old school websites that were, likely, built using invalid and/or outdated HTML code. It's a horrible feature, that I see no practical use for whatsoever. Plus it's easy to accidentally click it, because it's located a few pixels away from the refresh button--talk about non-intuitive...
This is how you disable IE compatability mode on your website.
In your .htaccess insert the following:
Header set X-UA-Compatible "IE=9; IE=8; IE=7;IE=edge"
Woooo! We're done. That dumb IE compatibility mode icon won't show ever again.