In contract bridge, avoidance play is a play technique whereby declarer prevents a particular defender from winning the trick, so as to eschew a dangerous lead from that hand.
The dangerous hand is usually the one who is able to finesse through declarer's honors, to give a ruff to the partner or to cash one or more established winners.
NS South plays 4♠ and West leads ♥K (indicating the ace), East playing the 3 (signalling the odd number of hearts and discouraging the continuation).
West continues with a club, increasing the probability of defensive ruff in that suit.
Since ♥A is almost certainly with West, if East gets the trump queen, he could finesse declarer's ♥Q.