A WordPress child theme is a theme that inherits the functionality of another theme, called the parent theme. Child theme allows you to modify, or add to the functionality of that parent theme. A child theme is the safest and easiest way to modify an existing theme, whether you want to make a few tiny changes or extensive changes. Instead of modifying the theme files directly, you can create a child theme and override within.
Our themes already come with a child theme ( Theme_Name-child-theme), in which you have some comments about the replaceable functions.