What is inducer in the lac operon? How does it ensure the "switching on" or of genes?

Explanation

Regulator gene, Promoter gene, inhibitor gene, Structural gene constitute an operon. The inducer is the lactose in the lac operon. When an inducer i.e. lactose is present in the lae operon, the represser joins with the inducer forming represser inducer complex. This complex prevents the represser from joining with the operator gene which is thus turned “ON”.
Jacob and Monad found that with the addition of lactose to. the culture, three structural genes (z, y, a) induced the formation of three enzymes-P galactosidases, permeases and transacetylases to break down lactose to glucose and galactose. The functioning of these genes is controlled by operator genes. The operator genes are turned "ON or OFF. by a protein termed represser produced by regulator gene.
(a) In the absence of inducer

(b) In the presence of Inducer

In an inducibile system, the repressor normally unites with operator gene and turns in "OFF" and this terminates the transcription. When an inducer is added to the culture medium i.e., lactose, the repressor joins with the inducer forming the repressor-inducer complex and this complex prévents the repressor to join with the operator, genes and turned it “ON”. The transcription and consequently the translation starts thus synthesizing the enzymes. In an inducibile system, the repressor normally unites with operator gene and turns in "OFF" and this terminates the transcription. When an inducer is added to the culture medium i.e., lactose, the repressor joins with the inducer forming the repressor-inducer complex and this complex prévents the repressor to join with the operator, genes and turned it “ON”. The transcription and consequently the translation starts thus synthesizing the enzymes.
Actual inducer of lac-operon is allo-lactose, a derivative of lactose. It is itself not metabolised such inducers are called gratuitious inducers.