Before schema.org came out there was another possibility to add structured data to a website: microformats.com. The website offered a really simple way of adding structured data: CSS classes. So every single theme author was able to add structured data by just adding CSS classes to HTML-Elements. However in 2017 we have schema.org which is much wider supported by many search engines. And now we don’t longer need these microformats. Unfortunately some older (and even new) themes still add those microformat CSS classes. The plugin can try to remove some of them for you so that it does not confuse search engines when you use schema.org.
The hentry class is one of the so called “microformats” and it was used to show search engines where they can find the main content of a page.
However there were even more microformats like the “vcard” that was used like a (as the name suggests) business card and to tell search engines more about the people on the website (like the author).