There are occasions when it is useful to play three notes where normally only
two would fit. The sign for this is a "3" placed over or under the group of notes.
Sometimes the sign has a bracket or a curved line as well.
So the definition of a triplet is three notes played in the time of two notes of the same kind.
Other combinations of "tuplets," as they are often called, are possible, for instance a
2 indicates that two notes are to spaced out over the time of three notes of the same kind.