Fluttertonguing (or flatterzunge) is a technique used by players of wind instruments (woodwind or brass), where the tongue is "rolled" - as though pronouncing a long, drawn-out "r" sound. It sounds like lots of rapid repetitions of the note, rather than a single long note.
Fluttertonguing can be used to create lots of different musical effects, from raspy brass snarls through to delicate mysterious flutes. It is moderately easy to do, but moderately difficult to do well. I happen to love the sound it makes, but your mileage may vary.