Humans.txt is an interesting idea - it's (another) way for websites to create an association between the people (developers, authors, marketers, and so on) that are behind the creation of the website and the website itself, without actually including that information in the content of said website. It's not exactly a web development standard, but it's a neat idea; and it's the kind of information that's "there if you're looking for it" but remains otherwise hidden from view - as it should be.
The format itself is very simple - it's a plain-text file, called 'humans.txt', and it's placed at the root of the website. In to that file you place whatever content you want, and whatever content is appropriate. A little more elegant than placing that content in the comments of your HTML, as has been done in the past.
For us Geeks, there's a neat little plugin available for Google Chrome, which simplifies the discovery and viewing of these humans.txt files, when they exist on any website you visit.