What is indicated by ‘r’ in the figure?

 In lac operon , i gene is inhibitor gene. i gene undergoes transcription  and forms mRNA , mRNA undergoes translation and forms reppressor protein ,in absence of inducer repressor protein binds to operator region and makes it OFF mode and in presence of inducer repressor binds with allolactose form of inducer and turns into ON mode. So repressor protein playing an important role in gene expression.