Hello,
Child theme is a theme that inherits the functionality of the parent theme. With a child theme, you can make modifications or add additional functionality to the parent theme. The child theme offers the safest and easiest way to modify an existing theme, whether you want to make a few small changes, or extensive ones. Read more – https://codex.wordpress.org/Child_Themes
You need to have both themes on site, the child theme should be activated.
Feel free to ask if you have any other questions.
Regards