Definition
Picking ahead is drawing your next tile from the wall before the player ahead of you has discarded, which speeds the game but removes the chance to call that discard.
It is common in casual games and prohibited in tournament play and by many groups, because a player who has already picked has lost the right to call the tile just discarded. Ask before you do it at a new table.
Read the full guide: American Mahjong Etiquette: The Unwritten Rules of the Table.
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.