Developer is a person who creates or edits various aspects of Moodle code.
Moodle is Open Source and has many tools to assist and coordinate developer's efforts. There is a section in MoodleDocs which is devoted to Developer documentation. The MoodleDocs wiki has a namespace called Development and also uses a Developer category. There are many Moodle forums where developers participate, including Section 4. Developers also use Tracker that is used to track bugs, improvements and suggestions for new Moodle features at http://tracker.moodle.org.
Developer documentation - a good starting point for developers