Why use the back to site button?
The Back to site button in the black admin toolbar provides an easy way to return to the last non-administrative page you were on after performing administrative tasks on your website.
Compared to the overlay that was previously used for administration pages, using the Back to site button improves the performance of the website and is more accessible.
How to use the back to site button
When you view an administration page, the page will be displayed normally in the full content area of the website and the Back to site button will appear in the top left corner of the website in the administration toolbar. The administration pages have the same look and feel as they did when using the overlay, which is different than the look and feel of the content pages of your website.
For example, if you click the Add content link in the grey shortcut toolbar, you will be taken to a page that looks something like this (the content types available vary by site):
To return to the last non-administrative page you were on, click the Back to site button in the top left corner (outlined in red in the screenshot above). Clicking the Back to site button has the same effect as clicking the "x" to close the overlay in the previous method of navigating between administration and content pages.