Undotted notes normally divide into two equal parts. For example, a quarternote is worth two eighths, a halfnote two quarters. If you want to play three notes that divide evenly the time of a quarternote, that would be called a triplet. Three triplet eighthnotes equal one normal quarternote, a halfnote is worth three triplet quarternotes (or one triplet halfnote plus one triplet quarter, etc.) If you select notes whose total value equals that of three eighthnotes and modify them with the Triplet button you'll find that they now are played in the time of a single quarternote.
If the notes you selected add up to a value that cannot be converted to a triplet the Triplet button will do nothing.
Tuplet is a general term for any division that goes beyond the triplet; for example, the quintuplet, sextuplet, etc. This button is far more general than the Triplet button because it also allows you to choose the equivalent time value of the group; for example, 7 to be heard in the time of 4 (triplets are 3 heard in the time of 2).
Once you've made a triplet or a tuplet you can adjust its appearance. Just as with slurs and ties, when the first note of the triplet is selected the triplet graphics become moveable: grab handles will appear on both ends of the triplet beam and a selection rectangle will appear around the number. You can move the bracket and the number as needed, but of course be careful to keep them on the notes they are intended for! Also, the Note Information window for the starting note will include a button to access additional controls for the triplet or tuplet. You can use this options window to specify, for example, that the triplet is to be shown with a number but not a bracket, or with both bracket and number hidden.