https://www.quora.com/What-does-this-tag-mean-html-lang-en-US-prefix-og-http-ogp-me-ns
https://stackoverflow.com/questions/27943451/meaning-of-html-prefix-attribute-open-graph-protocol
prefix is one of the attributes defined by the RDFa (Resource Description Framework in Attributes) extension. RDFa is used to implement the Semantic Web in web pages represented in many markup languages, like HTML and XML. Instead of having a web page telling the browser just how it should be structured, now you can also tell it what the page represents, like a Person, a List of Products, etc.
And for you to be able to semantically represent data on your web page, you need to use a semantic vocabulary, which tells the browser exactly that: what's being represented in your page. A popular semantic vocabulary is schema.org.