A useful technique for declarer to gain an extra trick is to “Finesse”. To finesse you need a high card then miss a card then the next card or better the next few. AQ or 108 or KJ109 for example. These cards can be in hand or in dummy or a combination of the two.
Say the AQ is in dummy. If you lead a small card towards the AQ then if the LHO has the King, it is trapped. If he plays the K you will take the A and the Q is the next highest. If he doesn’t play it you will finesse by playing the Q which will win and the A will win also so you will win two tricks.
Say you have the Axx in dummy and QJx in hand. Lead the Q and you are guaranteed two tricks (but not three) If the LHO rises with the K you will win the ace and the Jack. If he doesn’t you play low in dummy and the Q wins, then the A will win but no defender will let you win the jack.
Of course if the King is with RHO the queen will be taken by the K and only the A will win. The Finesse fails All things being equal the finesse will work 50% of the time.
Things are rarely equal however if you are going to finesse LHO for the King of Spades and RHO has bid spades twice he will have at least 5 spades probably six. Therefore, he is much more likely to have the K, similarly if the RHO opened 1NT then he will have more points than the other opponent and is therefore more likely to have the K.