This is a holder for a tritium vial (the vial itself does NOT come with the pendant). You can read all about tritium vials here, but, in short, they do NOT need to be exposed to sunlight to make them glow in the dark, and will last for many years.
This is designed for a vial that is 3mm by 22.5mm available here: http://www.eclipsesharp.net/collections/tritium-vials
You actually could use a smaller vial if need be, but no larger, of course.
The vial slips inside the top opening and your chain/leather necklace then keeps it in place.