The Ruby Glass windowpane. The sooty red oil lamp.
In short, the enchanted lantern of little light that has barely illuminated the photographer's cave since the earliest days. The dim signal that chemical magic was afoot. The safelight.
Let me start out by saying safelights aren't. None of them are safe, but their degree of danger depends upon a number of factors. The ones that were fired by kerosene were dangerous as fire hazards and the ones that work with electricity are dangerous because of that. But that is only to the worker - their real danger is when they overspread and fog up sensitive emulsions and photographic coatings. They are not meant to, but eventually they all do.
You defeat this by four means:
a. Distance - you keep the safelight far enough away from the sensitive material.
b. Time - you expose the paper or film for the very smallest amount of time needed.
c. Intensity - you keep the light as dim as you can, while still benefiting from some illumination.
d. Filtration - there are different filters for different emulsions.
This A-P safelight is a beauty for general use in black and white printing. It cannot be used to load panchromatic film ( and that's nearly all the ones you'll encounter as an amateur shooter ). You need to load films into holders and into tanks in dead darkness.
But it is fine for regular printing papers as long as you observe the time and distance rule. You can check yourself out with this by putting a coin on top of a sheet of printing paper in the dim red light of your darkroom and twiddling your thumbs for five minutes. Then develop the paper and see if you can make out an outline of the coin. If you do, reduce the time of exposure or move the safelight further away from the paper.
The beauty of these A-P lights as opposed to the red Edison-base bulbs we used to get is they are really just fridge lights in a holder - and you can get spare little globes at Bunnings.
And if you put one on your front porch of a night you can make a whole new set of friends.