the lac operon is required for metabolism (breakdown) of lactose. it contains a promotor, an operator, and some genes. the promoter is where RNA pol binds when we want to transcribe the genes. the operator is where our repressor protein binds when lactose isn't around--it binds there so that RNA pol won't transcribe our genes. the repressor just chills on the operator unless lactose becomes available and then it falls off and lets RNA pol transcribe the genes.