Child Themes is a feature available on WordPress. A child theme allows making modifications without impacting the main theme. In other words, it's very useful if you need a testing environment for any changes you make to your website, as your Main Theme will be unaffected by these changes. It's also very nice if you need to make a few different version of the same theme to see which one's better for you.
To find out more on what Child Themes are and how to set them up, try checking out this easy-to-use step-by-step guide: https://www.hostinger.com/tutorials/how-to-create-wordpres-child-theme