The functions.php file is a file that can be ioncluded in anything theme you might create for WordPress. This file is used to add unique functions to your WordPress theme. The functions.php file you can add new functionality to your site and modify the regular functions of WordPress. You can also gain access to many features of WordPress that are not available through the admin panel.
To use functions.php just create a plain text file named functions.php and add it to your theme folder. From here you will code that file that adds features to your theme.
Note: Every theme will have it’s own functions.php file and the features added by that file will only be in effect when that theme is active.