An eighth note triplet is a group of three evenly spaced notes played within 1 beat. Eighth note triplets are indicated by three eighth notes with the number 3 written either above or below the group.
Sometimes the triplet has a bracket or a curved line around the number 3. The eighth note triplets are played with a third of a beat each. Triplets are easy to understand once you have heard them played.
This example uses a constant stream of triplets on the chords played by the right hand.
Here is a 12 bar Blues pattern which makes use of triplets. Notice the left hand part in this example. This bass line is common in a Country style known as Western Swing.