HTML is a markup language that uses a special syntax or notation to describe the structure of a web page to the browser. HTML elements usually have opening and closing tags that surround and give meaning to content but some HTML elements don't have closings tags like

<img src='image path'/>

This is opening tag

<>

This is closing tag

</>

And self-closing tag is

</>

HTML is the building block of a website like skeleton in our body and brick or stone in a house.
Someone says that HTML is a programming language but it's a misconception.
The meaning of programming language is to add logic into your program or software how it to work.
HTML has 6 types of heading elements from h1 to h6.
The web browser displays h1 as the biggest heading and h6 as the smallest heading.
Every HTML has special meaning as I described above.

Examples

<h1>hello world heading 1</h1>
<h2>hello world heading 2</h2>
<h3>hello world heading 3</h3>
<h4>hello world heading 4</h4>
<h5>hello world heading 5</h5>
<h6>hello world heading 6</h6>

HTML has paragraph element as well looks like this one

<p>hello paragraph</p>

Beside these elements HTML many more elements but not useful like these ones:

<b></b>
<pre></pre>
<code></code>
<abbr></abbr>
...

Some of the screenshots I've taken from #freecodecamp website wanna to include here.