The pound symbol (#) followed by some word is how you apply styles to a document element having the same ID.
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.
<p id="myparagraph">This is a paragraph I want to make bold</p>
So you need to look at your own HTML document to figure out what those selectors are referring to.