A child theme is a theme that inherits or gets the functionality and styling of a previous theme, known as the parent theme. Child themes are the best way of editing or altering an existing theme. You can read more about Child Themes in the WordPress Codex
Why should I use it?
You will want to modify this theme along the way. It is recommended that you use the Child Theme for this. This way, when the parent theme is updated, your modifications are preserved with the Child Theme. You don't have to create your own child theme, simply make use of the theme from the zipped file downloaded from ThemeForest.
Where should I place my CSS modifications?
All the custom CSS rules can be added to the style.css file from the child theme. You can access this directly through: WP admin panel › Appearances › Editor.