What is child theme? How to use child theme in WordPress?

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.