Definition
The dragons are the twelve tiles in red, green, and white (soap), four of each, with each colour tied to one suit.
On the card a D means a dragon and its colour tells you which one.
Read the full guide: American Mahjong Tiles, Explained: All 152, Suit by Suit.
See it on real tiles: the coach reads any hand and names its direction, free, at the free read. Members get the coach on call for their own games.
More terms: Pung, Kong, Quint, Pair, Charleston, Blind pass, Courtesy pass, Joker.