A child theme is a theme that inherits the functionality and styling of another theme, called the parent theme. Child themes are the recommended way of modifying an existing theme. Check out the WordPress Developer Handbook for detailed information about Child Themes.
If you only want to change CSS styles, use the Jetpack plugin's Custom CSS option in the Customizer instead. Use this child theme if you want to make functionality changes.
You can download the starter child theme (ZIP file) and customize it for your congregation.