In a simple way, metadata is data that provides information about other data, or data about data such as wikipedia . If you understand programming you can easily associate this with the concept of arrays with indices semantics.

For example, in an image, a datum referring to that image ( size, etc. ) can be used to create the information also referring to this image, through which any entity can know that the image x has y of size.

In a webpage, or in a HTML document that in principle has as main objective the construction of the skeleton of the website, the idea is the same, to state how the page is organized, saying where are the headings, titles, content and so on, using key words specific to each area of the document. In a word document, or any other word processor does almost the same thing, if not in the whole - XML .

Another example could be the tags you used when creating this question:

  

html and feature-language

I, even before viewing the question, was able to know that it is related to html , and when viewing the content of the question, I saw "words" in bold, with a larger font than the rest of the page, and I was able to know what the title was, and when I saw the text with regular font below the title I also knew that it was the content, at the end, when I read it, I could see that it is really related to html .

If the tags were wrong, even though I read the content (which deals with C) , I would be confused because, imagine: I came here waiting to find content about html, but I found something different, how do I deal with it? . It would be worse if the question did not have any tag , I could clearly see that it's HTML so it's linked to the title, and the question is on the main page of the site, but someone browsing the tags HTML would have even found the question?

Although an example with people, when talking metatags refers to the digital world.

  

I do not know what the metadata means. And I would like to know what   is metadata relative to HTML documents?

  

As I explained what the metadata is, I'll explain what it is for in HTML documents.

HTML documents can serve a number of different purposes, with virtually everything you view on the web, assembled with HTML.

The metadata is in the header of an HTML page and is the first information read about the document. You can bring information such as page title , markup language version (! DOCTYPE ), in meta name="author" ) etc.

  

What is the importance of this metadata for an HTML document   (web pages)?

  

They are very important because the browser and search tools constantly use this information to improve the user experience by bringing more concise search responses and differentiating the browser tabs by titles and favicons that represent your site.

  

I do not know what the metadata means. And I would like to know what   is metadata relative to HTML documents?

In a very short time, the metadata is used to display relevant information from your site in a public and standardized tag for each information, for example, if you wanted to expose who is the developer or team that created the site, use this tag:

<meta name="author" content="Edvaldo Farias" /> 

  

What is the importance of this metadata for an HTML document   (web pages)?

It is very important for several services, but the main reason nowadays is referring to search engines such as Google, bing and etc. Metadata helps you identify important information easily, quickly, and standardized.