Red-light district

What is the origin of the idiom "red-light district" and when to use it?

The idiom "red-light district" originated in the late 19th century, primarily in reference to the districts in major cities where brothels and other adult entertainment establishments were concentrated. The term is believed to have originated from the practice of using red gas lamps to signify the locations of brothels. These red lights were used as a signal to indicate the presence of such establishments.