The note D is written in the space under the treble staff.
Play the note D with the second finger of your right hand.
This is a half note. It lasts for two beats. There are two half notes in one bar of * time.
This exercise contains four bars of half notes using C and D.
The big numbers 1 and 3 indicate that you play the note. The small numbers 2 and 4 tell you to hold until the next note.
Exercises 6 and 7 use a mixture of half and quarter notes.
Because of the * time signature, there must be a total of four beats in each bar.