avant qu'ils ne partent

 

Brad, 

 

this is the ‘ ne explétif ‘.

The attached links cover this. There is also a link specifically on its use with ‘ avant que ‘. The ‘ ne explétif’ is often dropped in speech, although still used in formal speech and writing.  (The first 2 links are to separate pages despite the same link description showing for them)

What is the ne explétif and when to use it in French

What is the ne explétif and when to use it in French  

https://www.lawlessfrench.com/grammar/ne-expletif/

Avant que + ne explétif + the subjunctive mood (Le Subjonctif) = Before I do in French