A functionality plugin is a way to separate what you might normally place in a theme’s functions.php file, and put it in a plugin instead. It works the same way as a theme functions.php file, but is separate from the theme and so not affected by theme upgrades, or tied to the theme so you loose all of your functions if you choose to switch themes.
This plugin automates the process of creating a functionality plugin. Simply install and activate this plugin, and your very own functionality plugin will be created for you. You can then edit your functionality plugin and add snippets to it using the quick link in the admin menu.
See this WP Daily post for more information on functionality plugins.
This plugin’s code is available on GitHub. Please feel free to fork the repository and send a pull request. If you find a bug in the plugin, open an issue.
Serbian translation provided by Ogi Djuraskovic from FirstSiteGuide.com.