HTML is a format that translates text into a hierarchy of special objects, called elements, that can be arranged into Web content. The specification is controlled by the W3C, who just promoted HTML 5.1 to “W3C recommendation,” which is their final stage for a standard excluding errata or a wholly new version.
Because standardization, intentionally, takes a very long time, this is not about new features or anything like that. In fact, one of the changes that I found interesting was the removal of appCache. This feature was originally designed for web applications to operate offline by ensuring everything it needs is stored locally. It wasn't really surprising, since Firefox actually warns users that it's deprecated since version 44, but notable none-the-less. (If anyone is wondering, Service Worker API replaced this API. Yes, I am aware of the Web standards joke “there are two standards for everything, but one is deprecated and the other is experimental”.)
If you're interested in just the changes, they are summarized here.