It contains a style guide like the one on his site which lists all the different elements that could be used on a site (paragraphs, headings, links…) and explains when and where you could use them.
There's also a section which is forked from Jeremy's Pattern Primer. This lets you show modules, such as error messages, pagination and comment forms, alongside their snippets of code.
You should try out Barebones in your next project. I've added this and a few other links to my front-end style guide stash if you're looking for more inspiration.