Moodle is a modular system.
This means that Moodle was built to be able to include additional functionality to its core platform. The Moodle Plugin Database is where official plugins can be searched and downloaded.
Moodle plugins allow users and organisations to extend and customise the functionality of Moodle beyond what Moodle HQ have developed for Core. This flexibility is what makes Moodle collaborative and community-enhanced.