Phosphorescence
Phosphorescence is the glow some materials give off in the dark after soaking up light.
Think of it this way
A glow-in-the-dark sticker on a bedroom wall soaks up light from the lamp. After you turn the lamp off, the sticker glows softly in the dark. It stored the light energy while the lamp was on and now slowly releases it. The glow fades over time as the stored energy runs out.
Explaining phosphorescence by grade level
You know how glow-in-the-dark things shine after you turn the lights off? That glow happens because the material stored up light while the lights were on. It holds that light energy inside, then slowly lets it out as a soft glow. The longer it sat in bright light, the longer it glows in the dark.