You may have seen the above string in Chrome developer mode.
It is a variable of the command line API. It stores what is selected in the "Element" panel or "Profiles" panel.
"$0" is the currently selected item.
You can check $0 at the console with the "console.log($0);".
There are five such variables($0 to $4). For example, $4 is the item you selected before the fifth.
※ More information on APIs in developer mode can be found here(Link).