So the question is, what is the semantic web? almost everything on the interest is being measured and in physical life too. When it comes to the internet, it is doing the same, like all the companies of the internet such as Facebook, Google, and Apple have agreed to work with a standard which in the internet case is Schema.org which provides the information to the robots of all search engines to identify and show the relevant results to the users. 

In a nutshell, semantic web is just the internet working on the one standard (schema markup)