The concept of “tempo” or timing is a very important one. The play part of a hand is a race between the defenders and declarer: the defenders are attempting to take enough tricks to beat the contract before declarer can take enough tricks to make her contract.
Tempo refers to gaining and losing the lead. The defenders start out with the advantage in tempo as they get to make the opening lead. To illustrate this, suppose you are defending against a contract of 3NT and you have to make the opening lead.
1) If you hold ♦AKQJ10, you can beat the contract immediately by taking 5 diamond tricks.
2) But suppose, instead you hold ♦KQJ109 and declarer (or dummy) holds ♦A. Now you can lead the suit, but declarer will obtain the lead as soon as she wins her ace. She may then be able to win an additional 8 tricks before losing the lead to your side. Thus she will make the contract.
3) However, if she needs to lose the lead to you — perhaps you hold ♣A and she needs to establish club tricks to make enough tricks — then when you win the ♣A, you can win your remaining diamond tricks to defeat the contract.
In the first and last case, the defenders had the advantage in tempo. In the second case, declarer had the advantage.
This may appear academic, but it isn’t. It is terribly important. To see this, suppose you have the same hands as in (1) and (3), but your partner has the opening lead not you. If, just for example, your partner who cannot see your lovely diamond suit, leads the ♥3, then declarer is going to make her contract. Your side started with the advantage in tempo, but lost the advantage because partner lead a heart and declarer gained the lead.
The best leads, both against notrump and suit contracts, are those that are most likely to take advantage of tempo. The very best leads are those that require surrendering the lead to declarer the least often in order for your partnership to win enough tricks to defeat declarer’s contract.