The theme functions file, or functions.php is the template used by all WordPress themes. It works like a plugin and is automatically loaded into the front-end pages and admin areas of a WordPress website. This file is usually used to define classes, filters, actions and functions used by the other templates of the theme. You can also use it to add additional features to your installation and theme, as well as extend its overall functionality.
To find the functions.php file, you will have to look in your theme’s folder. You can have both built-in functions and regular PHP functions added to predefined filters and hooks of your WordPress core. While every theme you install will come with a new functions.php file, only one file (associated with the theme you are using) will run at a time.
Child themes have their own unique functions.php files. These files can be used to expand on the parent theme functions or replace them entirely. Some of the most common features provided through a functions.php file include, the ability to enable post thumbnails, add navigation menus, post format options and use more standard WordPress Actions and filters.