The pound symbol (#) followed by some word is how you apply styles to a document element having the same ID.
<p id="myparagraph">This is a paragraph I want to make bold</p>
The words "page", "body" and "content" do not have any special meaning. They are just referring to something (which could be anything) you have identified by those words in your HTML.
So you need to look at your own HTML document to figure out what those selectors are referring to.