WordPress websites sometimes need to be customised so that the design, structure & style are changed.
Often the WordPress theme customiser can handle these customisations.
If you find that your WordPress theme customiser, custom CSS or Theme Options cannot handle the changes you require, then it’s time to create a WordPress child theme.
A child theme inherits every template and style from the parent theme, but allows you to store your own custom templates or styles so they override the parent theme.
Related: How to create a WordPress child theme.