In music notation, a triplet is indicated by a number '3' drawn above or below three notes joined by a beam, usually with a bracket spanning the three notes. It indicates that the notes indicated are to be evenly subdivided into thirds of a beat rather than halves. If three connected eighth notes are labeled as a triplet, then each one receives one-third of a beat instead of one-half; if three sixteenth notes are so labeled, each receives one-sixth of a beat rather than one-fourth.
A series of triplets on the staff looks something like this (complete with time signature):
/\
---| /------------------------|----------- 3 ----|----
|/ * | |\ |
---/--------------|--------*--|--|-\-----|\------|----
/| 2 * | *| | | | | |\ |
-/-|/\---------|--|----*|-|---|--|--|----|-|-|---|----
| | | 4 | | | | | | * | | | | |
|--|--|--------|-/----|-|-|---|-----|---*--|-|---|----
\ | | |/ | |/ | * * | |
--\|_/----------------|/------|-------------*----|----
| 3
\|