The lac operon is a set of genes in E. coli that helps the bacteria metabolize lactose, a sugar found in milk. It consists of three structural genes: lacZ, lacY, and lacA, which code for proteins that break down lactose into glucose and galactose. The operon is regulated by the presence or absence of lactose and glucose, allowing the bacteria to efficiently use available energy sources. When lactose is present, it binds to the lac repressor, a protein that normally blocks the operon. This binding changes the repressor's shape, preventing it from inhibiting the operon. As a result, the genes are expressed, enabling the bacteria to utilize lactose for energy.