What is RNAi based loss of function screening?
 

I'm currently trying to understand a Nature article and can't find a definition for RNAi based loss of function screening that makes sense.

This is the context:
Cancer cells adapt their metabolic processes to drive macromolecular biosynthesis for rapid cell growth and proliferation. RNA interference (RNAi)-based loss-of-function screening has proven powerful for the identification of new and interesting cancer targets, and recent studies have used this technology in vivo to identify novel tumour suppressor genes