To switch on or illuminate a red lamp or red light source.
2.
Generaltechnicalspecialized
In photographic or darkroom practice, to turn on the red safe light used to avoid fogging light-sensitive materials.
3.
UKUSEuropeGeneralinformalcolloquial
To use or display a red light as a signal that suggests the availability of sexual services or to indicate that an establishment is a brothel; used figuratively to mean to solicit clients for prostitution.