In the card game of contract bridge, to duck (or ducking) means to play low to a trick to which one has led, losing it intentionally in order to set up a suit or to preserve a control or entry.
South is on lead and if he plays to the ace and then the king and another, East will win the third trick.
Then, when the lead is regained, playing to the ace (or the king if the ace was taken before ducking) will establish the suit and the remainder of the cards can be cashed.
Note that defenders can benefit by a hold up play.
North, as a defender with (A K 4 3 2) and no outside entries, may do well to hold up on the first or second round of the suit, especially in a notrump contract.