Apple - Why does =$0 appear in Safari web inspector?

More than likely, you've used the Inspection Tool and have an element highlighted.

The $0–$4 variables contain the current and previous selected nodes from the Elements Tree Hierarchy.

source: https://webkit.org/blog/829/web-inspector-updates/

Chrome, Firefox, Opera, Safari - If you have an element currently selected in your "Elements" tab, you can use the reference $0 to call it within your code. For example, to see the elements' contents you would type $0.innerHTML. In Chrome and Safari, you can call the console from any of the other tabs by pressing "Escape" so you don't have to keep switching.

source: http://www.andismith.com/blog/2011/11/25-dev-tool-secrets/