A WordPress child theme is a theme that inherits the functionality of another theme, called the parent theme, and allows you to modify, or add to, the functionality of that parent theme. The child theme article on the WordPress Codex shows how to create a basic child theme and explains what you can do with it.
Making a child theme is very simple. Create a directory by using FTP or whatever file management application your host provides. Child themes are the primary way to add front-end functionality and presentation modifications to Layers.