Sometime you need to extend the theme myou bought with custom functionality or styles, but you can’t include you changies directly into the theme files, as each time you update the theme you will loose your custom code. To be able to write custom code and update theme without your custom data loose you need to switch to child theme. Child theme overwrites parent theme.

If you want to add custom styles switch to child theme and edit style.css of child theme. If you need custom functionality to include, again switch to child theme and add your functions to functions.php of a child theme. If you need to overwrite parent theme file, copy that file and put inside child theme folder (if file is nested inside folders, also create parent theme folders in child theme). Now edit copied file and you will not loose data when updating parent theme, and you will keep your custom data.

More about child theme you can find in wordpress.org