A triplet is a group of three notes played in the same time as two notes of the same kind. E.g., an eighth note triplet consists of three eighth notes played in the same time as you would play two eighth notes.
Eighth note triplets are indicated by three eighth notes grouped together by a curved line and the numeral 3, as shown above. Triplets are easy to understand once you have heard them played. Listen to the recording.
On the recording there are five beats to introduce this song.