A triplet is a group of three notes played in the same space of time as a group of two notes of the same note type. There are three different types of triplets in common usage, each with a specific identifying symbol written above the note group.
Exercise 37 introduces the most commonly used triplet type, the eighth note triplet.
Exercise 38 introduces the sixteenth note triplet. The first and then every alternate beat is counted, meaning either the right or left hand (whichever plays the first beat of the six note grouping) coincides with the count each time, e.g.: