Flutter tonguing is simply rolling your tongue rapidly like with the Spanish R while blowing out.
It makes a fluttering sound on/off of the note.
About this exercise#
Flutter tonguing adds a rapid, rattling buzz to a note by rolling the tongue against the roof of the mouth. If you can roll the R in a Spanish word, you already have the motion; you just do it while blowing steadily into the ocarina. The airflow keeps the tongue trilling on its own, so the sound flickers on and off many times a second. Some players find the rolled R impossible and use a growl in the throat instead, which gives a rougher but similar effect. Keep the breath even underneath, since the flutter fades the moment your air pressure drops.