An action card is a community card that is likely to generate a lot of action in the pot as it helps improve two or more players involved in a hand.
The type of card referred to as an action card will, for example, give one player a set while completing another player’s straight or flush. It usually results in several bets going into the middle.
A good example of this would be a player with J9 going against a player with pocket tens on a flop of 782, rainbow. A ten (T) coming on the turn would be an action card, as it completes the first player’s gutshot draw while giving the other player a top set.
In this scenario, Player A has the nuts. Player B has a very strong hand as well, with the option to improve on the river even if he’s up against a completed straight, so it is very likely for all chips to end up in the pot.
An action card doesn’t always have to improve a player’s hand. It can also be a draw-completing card that a player will start bluffing on, representing the type of range that the action card would help improve.