Fille is someone who is totally new to a game. Or seems like he/she is new to the game. It is kind of a word for noob, b00n, nab, newb etc. Just worse. In CS when a player has 0-21 score he/she is a Fille. A Fille might claim that he or she is totally Uber 1337. However he/she is not.
The word Fille origins from danish language and means "poor".