what is rel stylesheet in html code example

Example: rel="stylesheet"

The REL attribute is used to define the relationship between 
the linked file and the HTML document. 
REL=StyleSheet specifies a persistent or preferred style while 
REL="Alternate StyleSheet" defines an alternate style. 
A persistent style is one that is always applied when style sheets are enabled. The absence of the TITLE attribute, as in the first <LINK> tag in the example, defines a persistent style.

A preferred style is one that is automatically applied, 
such as in the second <LINK> tag in the example. 
The combination of REL=StyleSheet and a TITLE attribute specifies a 
preferred style. Authors cannot specify more than one preferred style.

An alternate style is indicated by REL="Alternate StyleSheet". 
The third <LINK> tag in the example defines an alternate style, 
which the user could choose to replace the preferred style sheet.