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.

Why use a Child Theme?
If you modify an existing theme and it is updated, your changes will be lost. With a child theme, you can update the parent theme (which might be important for security or functionality) and still keep your changes.
It can speed up development time.
It’s a great way to get started if you are just learning WordPress theme development.