In so-called simple time each beat is naturally subdivided into two halves (and 4 quarters, 8 eighths, etc.)
The time signatures of 4/4, 3/4, 2/4 and 2/2 are examples of simple time.
In compound time the beat naturally subdivides into three.
The time signatures of 6/8, 9/8, and 12/8 are examples of compound time.
Occasionally a group of three notes is used in simple time to fill a beat. In such a case the time signature is not changed, but the notes are grouped in threes with a figure 3 above or below them. This group is known as a triplet.
The illustration below shows the position of three triplet quavers in relation to two ordinary (half beat) quavers filling the same beat.
Each triplet quaver is two thirds of the length of a normal quaver.