In fact, "ette" is a suffix that we sometimes add to some nouns, and its meaning is "small".
So "une fillette" is a small girl, a young girl. "Une maisonnette" is a small house. But actually we don't really use it in daily life, and we prefer to use the adjective "petit", also because this suffix tends to have a pejorative meaning. By the way, "fillette" in colloquial language is also used to "offend" (this isn't a BIG insult though) someone who's afraid of doing something.
For example, let's imagine a group of friends on a lake :
-Non, je ne veux pas aller dans l'eau, elle est trop froide pour moi !
-Oh, t'es vraiment une fillette !!
Hope it helped!