You now know notes using all the different letter names used in music - A B C D E F and G. Although there are only seven different letter names used for notes, these notes are repeated on higher and lower pitches. The distance between a note and it’s next repeat is called an octave. Practice alternating between the low and high D notes in the next example. These notes are an octave apart.
This song alternates between low D and all the other notes you have learned before arriving at the D note an octave higher. The order is reversed on the way back down.