A "tempo" is one turn's worth of time, and gaining one means making your opponent waste theirs. The cleanest way to do it: develop a piece to a square where it ALSO attacks something, so your opponent has to spend their move reacting instead of developing.
The classic example comes straight out of the Scandinavian Defense: Black recaptures on d5 with the queen on move two, and White develops the knight to c3 with an attack on that queen. White gets a free developing move; Black moves the same queen twice before developing anything.
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Black's queen came out early to recapture on d5. White can develop and attack it at the same time.
Develop a piece while attacking the queen.
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