What is a WordPress Child Theme?
A WordPress child theme is a theme that is used in conjunction with a parent theme and inherits that parent theme’s functionality. According to WordPress:
A WordPress 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 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 You Should Use Them
Basically, you get the benefit of the parent theme’s functionality but can add your own custom features and styles without having to hack the parent’s files. This protects your customizations against future updates to the parent theme. If you’re using a theme such as Genesis or Divi from Elegant Themes, or any other theme for that matter, you will want to use a child theme so you can safely update the parent when an update is available. If you were to edit the parent’s files, your edits would be overwritten when you update.