For example, if you are using the current default theme in WordPress, Twenty Seventeen, you might want to change what typeface the theme uses. If you make those changes directly to the Twenty Seventeen theme files, they will get overwritten the next time Twenty Seventeen is updated. Instead, you can add a child theme that inherits the style of functionality of Twenty Seventeen while enabling you to make small adjustments to its presentation. In that case, you’re using Twenty Seventeen as a parent theme.
When a child theme is active, it takes priority over the parent theme. That means that if both the parent theme and the child theme include the same template file, like the template for sub pages, the file included in the child theme will be used when the page is loaded.
Parent themes can be updated without you losing the changes you’ve made in the child theme. Changes introduced by updates to the parent theme could still introduce conflicts with the code in your child theme, if new functionality is introduced or old functionality is removed, but there’s no risk of you losing the updated you’ve made entirely.