what does hr stand for in html code example

Example 1: hr tag

<hr style="height:2px;border-width:0;color:gray;background-color:gray">

Example 2: which is right or in html

<hr> <!--   it represents a horizontal line break, in HTML4.0 we use <hr>, 
            in XHTML1.0 we use <hr/>,  in HTML5(which is the current version of 
            HTML) we can use both <hr> and <hr/>    
       -->

Example 3: hr tag html

<hr> <!-- A horizontal line -->

Example 4: what does in html mean

<p>HTML is a language for describing web pages.....</p>

<!--the hr tag stands for horizontal rule and will make a line across your page at that point-->
 <hr>

 <p>CSS defines how to display HTML elements.....</p>