In Lesson 4 you learned that a key signature can be used instead of writing a sharp sign before a note each time it occurs. Because the D major scale contains the notes F# and C# , the key signature for the key of D major will consist of an F# and a C# written after the treble clef.
This is the key signature for the key of D major. It has two sharp signs after the treble clef.
This piece is in the key of D major as indicated by the key signature. There are literally hundreds of songs which can be played using only the notes of the D major scale.