A triplet is a group of three evenly spaced notes played within one beat. Eighth note triplets are indicated by three eighth notes grouped together by a bracket (or a curved line) and the numeral 3.
Each note has the value of one third of a beat. Triplets are easy to understand once you have heard them played.
Notice the different sound used for counting each part of the triplet: 1 - trip - let.
Once you are comfortable playing triplets on one note, try running through a scale with them, and then play some sequences based on triplets, as shown in the following example. Use alternate picking and notice that one beat starts with a downstroke, while the next starts with an upstroke.