An operon consists of a group of genes which are controlled by a single promoter.
The lac operon present in the E.coli comprises of genes for making proteins which breakdown lactose into glucose.
Normally, the lac operon will be turned of by a repressor when there is not enough lactose present.
When lactose is present in the cell, the operon is activated when lacl forms a bond with the promoter region due to its changes in the structure. This causes the process of transcription to occur.
Context would be helpful but the general answer is that it is important to have the subjects of an experiment blind to treatments to reduce the effects of the placebo effect
