Open Graph Makes your website into rich "graph" objects.
Now, what I understand from it is that it provides us to add additional metadata to your website which makes it more rich on social media. For e.g you see the thumbnails of links in the Telegram - courtesy of OpenGraph
Information is added into the <head> tags. For e.g below is the metadata of one of my blogs
<metaproperty="og:description"content="A collection of C++ STL features (functions/libraries) which can be learned in 30 seconds or less"/>
<metaproperty="og:title"content="30 Seconds of C++"/>