Flutter tongue is an extended technique used on the flute to give an effect to the tone quality.
In essence, roll the tongue while blowing a flute tone.
This can be difficult. Like other extended techniques, it takes a lot of air.
I found this quite hard to do and just couldn't get it at all. Then a TV commercial helped me!
Do you remember the old "Ruffles have ridges" commercial? In that, they really emphasized the rolled R in "Ruffles" and "ridges." I just tried to imitate that and was able to do it.
For those of us who speak English as our first language, the rolling R is an uncommon feeling. Think of someone speaking English with an accent. French, Spanish, Middle Eastern. So many languages roll the R. Got the sound in your mind? Now try to talk with a rolled R. Now, roll the R and blow across your flute, trying to play a G. Once you've accomplished that, try to play a scale or a simple tune while fluter tonguing.
Another way to think of it is gargling. Try gargling first with water, then transfer the feeling while playing the flute.
Contemporary flute pieces will incorporate flutter tonguing, but if you don't play such works, flutter tonguing is a fun tool for learning to use more air with your flute playing.