Definition
Racking is placing a drawn tile onto your rack, and it matters because once the next player has drawn and racked, the previous discard can no longer be called.
This is the clock on every call. If you want a discard, say so before the next player racks. Holding a drawn tile in your hand rather than racking it keeps the table's window open, which is why groups ask you to rack promptly.
Read the full guide: Calling and Exposing in American Mahjong: When, How, and When Not To.
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.