I was reading BIOS Biochemistry and stumbled upon a phrase 'background level' in the following excerpt $-$
The lac operon contain lacZ,lacY and lacA genes encoding $beta$-galactosidase, galactoside permease and thiogalactoside transacetylase, respectively, preceded by an operator site and a promoter. The operon is transcribed by RNA polymerase to produce a single polycistronic mRNA that is translated to produce all three enzymes. When lactose is present the background level of $beta$-galactosidase converts some lactose to allolactose, which induces transcription of the lac operon.
What is meant by background level? Does it refer to the basal level of transcription, the constitutive transcription of facultative genes?