A Child Theme is a Theme that inherits the functionality of another Theme, called the parent Theme. Child Themes allow you to modify, or add to the functionality of that parent Theme. A Child Theme is the best, 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.
Child Themes – WordPress Codex