This is a vert simple little shortcode. It’s used to embed your tumblr account into a page on your site. And whilst it’s designed for tumblr it can also be used for other sites too as long as they support being embedded (if you’re not sure if they do, just try it and see what happens).
To get the most out of it, I suggest you add this custom theme to your tumblr:
Ensure infinite scroll is on too, for all that fancyness.