Child theme is a theme that inherits functionality, features, and styles from its parent theme. It’s present in all of our themes download packages and the purpose of having the child theme is to offer a way to modify main/parent theme features or styles without altering the code of the parent theme. In this way, you can always update your main theme without any worries regarding your customization and changes.
How to use Child theme
In order to use the child theme, you will need to install it right after you install your main / parent theme as the child theme is dependent on the main theme and it can’t work without the main theme already installed. So, once you install the main theme, just upload the child theme, install and activate it.
Now you should have your child theme up and running and if you want to alter some part of the main / parent theme files, just copy them into the child theme directory and recreate the same path for that file to be like in the main theme and alter the code. That code will override the main theme template file and you will have your customization up and running. Once you decide to update the main theme you will not lose any changes or customization made in the child theme directory.
You can read more regarding the Child themes on official WordPress codex page here: https://codex.wordpress.org/Child_Themes