Basically a pair of huge pigtails on a girl, that are much larger than possible in Real Life, at least not without loads of hair gel. The minimum hair size of both tails combined must be at least the size of the girl's head.
There can be many reasons for this, although the most common are either to make the pigtails clear even in long shots, or just plain Rule of Cute.
Quite often these pigtails [...] [tvtropes.org]