This new D note is one octave higher than the previous D note. An octave is the range of eight notes between a note and its next repeat higher or lower.
The range of notes on most instruments is divided into registers. The term "register" is used to describe a range of notes that have a similar tonal quality. This is something you will be able to hear more easily as your playing develops. The saxophone has four registers which are shown below.
D in the middle register uses the same fingering as D in the low register, but with the octave key added. The octave key is played by rolling the left thumb forward and up from the thumb rest. Practice this movement many times until it becomes automatic.