jameyer is correct. The hentry class belongs to the hAtom microformat specification.
What is a microformat?
"microformats are extensions to HTML for marking up people, organizations, events, locations, blog posts, products, reviews, resumes, recipes etc. Sites use microformats to publish a standard API that is consumed and used by search engines, browsers, and other tools."
What is hAtom?
"hAtom is a microformat for identifying semantic information in weblog posts and practically any other place Atom may be used, such as news articles. hAtom content is easily added to most blogs by simple modifications to the blog's template definitions."
However it looks like hentry/hAtom are part of Microformat v1. Microformat v2 has updated hentry to h-entry. Maybe it needs to be updated in the WordPress lib?
More about h-entry:
"h-entry is a simple, open format for episodic or datestamped content on the web. h-entry is often used with content intended to be syndicated, e.g. blog posts. h-entry is one of several open microformat draft standards suitable for embedding data in HTML/HTML5."
And finally some info on hentry: