A child theme is a theme that inherits the functionality from the parent theme and are often used when you want to make changes to the theme that goes beyond the default options. Changing code directly to the theme files from the parent theme is not possible because you will loose them when you update the theme, this is why installing a child theme will help you.
The child theme is included inside the “All files & documentation” package from ThemeForest. If you want to use the child theme upload it through the WordPress dashboard (zip file) or ftp (unzipped folder). Please see Upload Theme how to upload and activate the child theme