What is html headings?

HTML headings are defined with the <h1> to <h6> tags.
<h1> defines the most important heading. <h6> defines the least important heading.
 such as-
<h1> This is heading 1</h1>
<h2> This is heading 2</h2>
<h3> This is heading 3</h3> 
<h4> This is heading 4</h4>
<h5> This is heading 5</h5> 
<h6> This is heading 6</h6>