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.
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.