How does the right-hand margin work?

Using the blog theme, I did a short text-only post and noticed that the text all stays contained in the left side of the page. It seems to be a two-column layout, but I don’t know how to use that right-hand column. Could you explain how the layout works and how I can best use it?

P. S. This is the coolest software project I have seen in a very long time.