PocketMine-MP can be extended and customized with plugins.
A plugin is an external module which can be "plugged in" to your server to add custom features, change default behaviours and more.
You can find a wide range of premade plugins on the Poggit Release plugin platform.
Plugin developers usually distribute plugins as phar files. These files can be easily loaded by placing them in your plugins folder in the server data folder.
Be sure to delete old versions of the plugin, and restart the server to see changes.
You can find resources for plugin development at the DeveloperDocs site.