This nail polish can be charged with light, e.g. with blacklight from a UV torch, or with daylight. Afterwards, it glows in the dark for some time; a great effect, not only for Halloween!
Important information about the "Glow in the Dark" effect: you can only see the glow effect in a dark room if your are accustomed to the darkness. After being exposed to bright sunshine and going into a dark room, the eyes need some time to get used to the darkness, so you will not notice the glow effect immediately - maybe you will not notice it at all. The glow effect lasts for about 5-10 minutes (if the color is well charged with light), so it does not work to 'charge' the color during the day to see the light effect at night (the transition from bright daytime to nighttime takes far too long).
The effect works better when using blacklight lamps: these lamps quickly charge the nail polish with light, while the eyes are already used to the darkness. If you switch off the blacklight lamp or go into a dark room without blacklight you will be able to see the glow well.