Translation:I have to be at home before the night falls.
This is called the "Expletive Ne." It doesn't mean anything in the sentence, but in some sentences you need to add it. Here is a brief article that helped me: http://french.about.com/od/grammar/a/negation_form_2.htm
If you look at the usage at the end of the article, you'll see that an expression following "avant que" requires it (as in this sentence).