A district in which houses of prostitution are numerous.
Like a salmon swimming upstream to spawn, George makes his annual visit to the red light district in Amsterdam.
The phrase red light district is no doubt because of the fact that at least in the most famous of these, Amsterdam, they do in fact have red lights in the windows. This to let prospective customers know that it is a house of prostitution.
The term originated around 1900 and seems to have come about from visiting railroaders leaving their lanterns on the porch of the cat house while conducting business inside.
Red lanterns where required equipment for all railroad crew members, with the possible exception of the engineer. Prior to 1900, a train would likely have an engineer, conductor, fireman, and two or more brakemen (brakes being manually applied on a car by car basis). They were used for signaling as well as lighting, since red light does not affect night vision as much as white.
A train being brought into a yard would have its crew surrounding it, most carrying red lanterns to signal the engineer that all was well. They would then use these same lanterns to illuminate their path to wherever they chose to go.
And the question is why did the railroaders leave the lanterns on the porch? Apparently to indicate to other prospective visitors that the venue was in use.