The number under the notes are scale degrees. On the top right, you can see the notes in the D major key. For example, the F# note is the 3rd in the list from the left. You will notice the sequence contains F#4 first. Under it is the number 3. The scale degree is 3, because F# is the 3rd note in the key/scale. For example the scale degrees are D=1 E=2 F#=3 G=4 A=5 B=6 C#=7. If you were to change key, the notes would have different scale degrees dependent on their position in that new key.
