The Action card is a name given to a card that pops up on a flop, turn or river, and which leads to heavy betting from all players. This term is normally used in the Texas Hold’em style of play or other games where community cards play a major role. This is quite common when a card that appears may be of help to one or more players.
Let’s say there are three players left in a game – A, B and C. A holds a 4-7 in her/his hand, B has a 6-6 while C has a 2-7 in her/his hands. On the flop, the cards that turn up are 5-6-8. Due to these cards on the board, both A and B have a chance of winning and that results in a significant increase in the betting activity. C might also believe that s/he has a significant chance if a favourable card turns up on the turn or river.
Therefore, both A & B, and if C is daring then C as well, will be part of the betting activity caused by these three cards popping up. That means that 5, 6 & 8, the cards that turned up on the board, are the action cards.
In case the board had 7-T-J on the flop, then 7 would be the action card as both A and C have 7s in their hand meaning that they would begin heavy betting activity upon its appearance.