To decline to take a trick that one could have won. Ducking is often a good idea to avoid playing a high card on a low one, to leave the opponents with a guess or to disrupt the opposing communications. Ducking can also maintain your side’s communications:
If you are trying to set up the diamond suit when North has no side entry, you need to duck the first two rounds of the suit, conserving the ♦A as an entry to the long cards.