What is functions.php?
Functions.php is also known as the theme functions file. You can use this file to add new functions to any theme, which will activate while using it.
More About functions.php
Every theme in WordPress has its own folder in the wp-content/themes directory. Inside each theme’s folder, you’ll find a unique functions.php file. Child themes also have their own functions.php file, which takes precedence over the parent theme’s file.
Functions.php can include any type of customizations and features that you want to add to WordPress. However, any custom code that you add using a functions.php file will only remain active as long as you’re using the theme that corresponds to that file.
If you want to add new functionality to WordPress and not make it theme-specific, we recommend using plugins. Plugins are also the preferred option if you’re not comfortable adding custom code to WordPress files.