A modder is a person who publishes new or improved YS Flight Addons. The term is derived from modification and modeler. It is important that modders who use the works of other modders respect and follow the established guidelines for such actions. This generally requires obtaining permission from the original modder to create derivative works, and always requires crediting the original modder in the release documentation.
Modders are a critical part of the YS Flight community because they keep adding new matieral and refreshing the game for all players. While becoming an expert modder takes time and effort, it is not hard to begin modding. There are simple things that new modders can begin with like painting aircraft or creating basic maps. You can find tutorials on how to do so both here at the YS Wiki, YSFHQ, or on Youtube as some of the most experienced modders have created and published tutorials in all these places.