What is child theme, and when should it be use?

Child theme is a theme that’s working under the mother theme also called main theme. And changes added to the child theme overwrite the layout. And because WordPress is based on PHP that is a dynamic code this is possible

To explain what’s dynamic code is, I would need one full article. But to explain it short… when a request come on a WordPress site the code put the site together.
So when should you use a child-theme? The simple answer is, always use it when you need to make any changes to your theme code. With one exception, if you only make changes to CSS, you don’t need to bother

CSS are mainly an design code. With CSS you make changes to fonts, colors, spacing sect. ect. WordPress has integrated a feature where you can add your own CSS that remains even when you update the theme.

But these changes disappear, if you change the theme, and then you have lots of different plugins that can handle your CSS changes.

So if I have make changes, why do I need to bother?

Sooner or later you will need to update your theme. And to not update your theme it’s a security risk. But it can also create problems with updated plugins.

So: The main rule is, always have your theme updated and of course your plugin also. And if you made change to your theme, and add this changes to an child-theme you will not have any problem.

But if I’m happy with my theme as it is?

I always recommend to set up a child-theme when you have found a theme you are happy with.

If you have a theme with lots of custom settings or if you have done lot of settings in the “Customize” menu in WordPress. All these will be reset when you turn on your child theme.

Read more about how you create your child theme here