7
It is the element that is selected. It can be referenced in the Javascript console with the variable $0. For example (using jQuery), if you type this in the console:
$($0).html()
The HTML of the element you selected will appear (you can test it here in the Stack Overflow page). Of course this is not limited to calling html(), you can do whatever manipulations you want in the DOM using the $0.

Greetings, Lorena. I took the liberty of editing your question in order to clarify the question about
== $0. As for your other question, to keep the header fixed, I recommend you to open a new question, as it has no relation to this doubt.– Woss
Thank you so much. I’m sorry.
– Lorena Passagem