Child themes are separate themes that rely on a parent theme for most of their functionality. If you are using a child theme, WordPress will check your child theme first to see if a specific functionality exists. If it doesn’t, it will use the parent theme. This is great because it allows you to modify only what you need.
Child themes should always be used if you plan on modifying even a single character in your theme.
If you plan to edit the theme files, like style.css, we are suggesting you to install both sixten.zip and sixten-child.zip themes, then activate the child theme. Child themes are great because allow you to edit the theme and also lets you update the original theme in the future, without loosing your edited files work.
More about child themes and how to use them you can read here