Congratulations on your new 18-pipe tenor panflute! Treat it with love and it will give you joy for many years.
This tenor’s lowest note is middle D and it is tuned in the key of G. When I refer to key I am speaking of its tonal center or the first note of its major scale (Do Re Me).
However melodies do not always begin on the first note of the scale. Many melodies start above or below the tonal center. By changing the key of your panflute you can increase the choices of how melodies fit in the range of your instrument.
Later, after you have played for a while, you can experiment with some different keys. It is easy to do, just by pushing up or down on the little rubber stoppers inside each tube, you raise or lower the pitch.
You can use a wooden dowel to do this. You can even mark the dowel at different positions to make it very quick. Ask if you need help.