The lac repressor is a protein that plays a crucial role in the regulation of the lac operon, which is involved in the metabolism of lactose in bacteria like Escherichia coli. It binds to the operator region of the operon, preventing the transcription of genes necessary for lactose utilization when lactose is absent. When lactose is present, it binds to the lac repressor, causing a conformational change that reduces its affinity for the operator. This allows RNA polymerase to access the promoter and initiate transcription of the operon, enabling the bacteria to metabolize lactose efficiently.