Phosphorescence First, let's distinguish phosphorescence from fluorescence. A fluorescent paint glows under a UV lamp, but stops glowing as soon as the lamp is turned off. A phosphorescent paint keeps glowing for a while.
Phosphorescent substances have the ability to store up light and release it gradually.
The notion of a metastable state explains this. If the molecules of the substance can get from the ground state to a metastable state, and if the metastable state can slowly decay back to the ground state via photon emission, then we have phosphorescence. Typically, the metastable state is a triplet state, and the ground state is a singlet state. Most of them immediately hop right back to the ground state, emitting a photon, but non-radiative processes take a few to a less energetic triplet state. Once these molecules get to the lowest triplet state, they are stuck there, at least for a while.