Operons can be regulated by repressors or by activators.
A repressor binds to the operator to prevent the transcription of the genes in the operon, while activators bind to the operons to activate the transcription of the genes.
When an operon is expressed constitutively (this means, the default is that the operon expresses its genes), operons are repressible (can be stopped), and this can happen by adding a repressor (negative repression) or by removing the activator (positive repression).
An example of a repressible operon is the trp operon, which synthesizes tryptophan, an amino acid. It is usually transcribed until there is enough tryptophan and the repressor binds to the operon.
When an operon is expressed just under certain circumstances, is an inducible operon (can be activated), and this happens by removing the repressor (negative induction) or by adding an activator (positive induction)
An example of an inducible operon is the lac operon, involved in the metabolism of lactose, where the transcription occurs only when lactose is present, which removes the repressor and allows the transcription of the enzymes.