A technique used when playing the flute (and maybe some other instruments, though I don't really know). While fingering notes as normal, the tongue is flicked up and down extremely rapidly. This produces short little machine gun bursts of air across the instrument. So instead of sustained notes, you get an effect where about half the time the note is heard, and half the time it's not.

When you're supposed to use this technique, it will be marked on the music. Generally, just flutter your tongue in addition to doing everything else normally. It's hard to do this right and to me, it doesn't even sound very good.