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. Instead of modifying the theme files directly, you can create a child theme and override within.
You will find child Theme version in the package you downloaded from Themeforest
in the directory called CHILD-THEME.
More information about Wordpress child themes can be found on Wordpress Codex.