The theme that the child theme inherits its functionality and styling from is called a parent theme. The parent theme must be installed on the site for the child theme to function. A child theme can be used to customize the styling of the parent theme, add new functionality to it or make other changes that are limited enough in scope that you’d rather not create a new theme from scratch.
Using a child theme is the recommended way to make changes to a WordPress theme that you have installed from WordPress.org or from a third-party theme vendor. If you make changes directly to a theme from WordPress.org, for instance, those changes would get overwritten the next time the theme is updated. If you instead make your changes in a child theme, the changes will remain when the parent theme is updated.
If you only want to make changes to the styling of your theme, a good alternative to developing a child theme is to use the WordPress Customizer.