justbrowsingIt's a glow-in-the-dark tube. Just like normal glow-in-the-dark material, you expose it to light, and it continues to illuminate for a few hours after.
It is NOT a tritium vial, which ALWAYS glows from radioactive decay and phosphors coating the inside of the tube.
theoneandonlymdThat's what it does, not what it's for.
It's for hanging on your keychain or bag or coat or whatever if you're someone (like me) who has to regularly walk at night (i.e. walking to the train, etc.) and want to make sure you're seen and not squished.