what is child theme why use this in wordpressssssssssssssssssss code example

Example 1: how to make folder in child theme in theme editer

/*
 Theme Name:   WPB Child Theme
 Theme URI:    https://www.wpbeginner.com
 Description:  A Twenty Thirteen child theme 
 Author:       WPBeginner
 Author URI:   https://www.wpbeginner.com
 Template:     twentythirteen
 Version:      1.0.0
*/
 
@import url("../twentythirteen/style.css");
 
.site-title {
padding: 30px 0 30px;
}
 
.site-header .home-link {
min-height: 0px;
}
 
.navbar {
background-color: #e8e5ce;
}
 
.widget { 
background-color: #e8e5ce;
}
.site-footer {
background-color: #d8cdc1;
} 
.site-footer .sidebar-container { 
background-color:#533F2A
}

Example 2: create child theme wordpress

/*
 Theme Name:   Twenty Fifteen Child
 Theme URI:    http://example.com/twenty-fifteen-child/
 Description:  Twenty Fifteen Child Theme
 Author:       John Doe
 Author URI:   http://example.com
 Template:     twentyfifteen
 Version:      1.0.0
 License:      GNU General Public License v2 or later
 License URI:  http://www.gnu.org/licenses/gpl-2.0.html
 Tags:         light, dark, two-columns, right-sidebar, responsive-layout, accessibility-ready
 Text Domain:  twentyfifteenchild
*/