The lac operon has three genes that encode for proteins that break down lactose into galactose and glucose. When there is no lactose around, it wouldn't make sense for the prokaryote to make these proteins as it would be a waste of energy. So, when there is no lasctose around, the lac repressor binds the operator region of the operon, which inhibits RNA polymerase's ability to bind and start gene expression. However, when lactose is available, it binds to the lac repressor and deactivates it via allosteric inhibition. Once the lac repressor is gone, the genes can be transcribed into mRNA which can be translated into proteins which can go and break down the lactose.